Short answer: the most expensive single-component front-end parts used for 1.420 GHz (H-line) work are cryogenic HEMT/InP LNAs and the associated cryogenic front-end systems (LNA + cryostat + cryocooler). Top commercial cryogenic LNAs (Low Noise Factory / Quantum Microwave / Narda-MITEQ etc.) are quote-only and typically run thousands → tens of thousands of USD each, while a complete cryogenic receiver (with cryostat + compressor) pushes the cost into the tens-of-thousands to hundreds-of-thousands USD range.

Why — short breakdown with concrete examples

• Premium cryogenic LNAs (best single-device performance): manufacturers such as Low Noise Factory produce InP/HEMT cryogenic LNAs (models covering L-band and wider) with noise temperatures down to a few kelvin. These are sold by quote (no catalogue price shown), which is typical for high-end devices and implies prices in the multi-thousand USD range (and for specially configured modules can reach well into five figures). If you want the lowest noise at 1.42 GHz, this is the route people take.
• Cheap hobby/entry LNAs & filters: there are many sub-$500 LNAs and 1420 MHz bandpass filters aimed at amateur radio astronomers (examples on Tindie / RadioAstronomySupplies / GPIO show items priced from a few tens to a few hundred USD). Those give good performance for backyard hydrogen-line projects but can’t match cryogenic systems.
• Cavity / high-Q filters: commercial cavity bandpass filters tuned to 1420 MHz (used to protect the LNA from out-of-band RFI) are commonly available in the hundreds to low-thousands USD range (example: a 1420 MHz LNA cavity band-pass filter listed ~US$375). Extremely high-spec, custom cavity filters or cryogenic filters will cost more.
• Complete cryogenic front-ends (what makes the big jump in cost): adding a cryostat + cryocooler to operate the LNA at 4–20 K is what drives costs up. Cryostats and compressors are commonly quoted in the $8k–$30k+ region (examples: cryostat quotes in the ~$8k–12k band; cryocooler/compressor quotes commonly cited ~$23k for a single unit). When you add engineering, integration, and the premium cryogenic LNA itself, a full L-band cryogenic receiver front-end for a professional telescope commonly reaches tens of thousands — sometimes >$100k.

If you want a quick decision guide

• If your priority is absolute lowest system noise for hydrogen-line work → buy a cryogenic LNA + cryostat/cryocooler (expect quote prices, plan on multi-k$ to multi-10k$ or more for a turnkey front end).
• If your priority is low cost / easy setup for hobby hydrogen-line detection → a quality room-temperature 1420 MHz pre-filtered LNA and a cavity bandpass filter will typically do the job for tens → a few hundred USD.

Here’s a practical, ordered rundown of the most expensive (but still ≤ $2,000) LNA / filter options that an amateur backyard hydrogen-line setup would realistically consider. I ordered the list most expensive → least expensive and gave one or two concrete examples and current price points I found online so you can shop from them.

Quick note: true cryogenic LNAs / cryostats are well above $2,000 and so are excluded. The items below are room-temperature or surplus commercial modules and filters suitable for backyard use.

1) Surplus / professional rack / commercial LNA modules (used or new surplus) — variable, often hundreds → low thousands

• What: commercial LNA modules from suppliers/manufacturers (Narda-MITEQ, MITEQ, other microwave vendors). These are the highest-end room-temperature amps you’ll find before hitting cryogenics.
• Price evidence: there are many surplus/professional MITEQ / Narda items listed on marketplaces (examples show units listed in the low hundreds on eBay; new/rack units from distributors can be quote-priced and sometimes exceed $2k). Expect typically $100–$1,500 for many useful surplus modules, depending on model and condition.

2) Dedicated radio-astronomy 1420 MHz pre-filtered LNAs (purpose-built for H-line) — ~$200–$400

• What: LNAs sold specifically for hydrogen-line hobbyists (pre-filtered to reject RFI around 1420 MHz). Good compromise: low NF and built-in filtering.
• Example: RadioAstronomySupplies “1420 MHz ULTRA HIGH GAIN LNA” — ~US$225. Excellent, common choice for backyard rigs.

3) Higher-grade lab / component LNAs (Mini-Circuits, distributors) — ~$100–$350

• What: well-specified lab LNAs (Mini-Circuits ZFL family, other catalog parts) — slightly higher reliability and datasheet detail than the dime-store modules. Often used as front-ends when combined with a bandpass filter.
• Example price: Mini-Circuits ZFL-500LN (or similar) listed ~£100 (≈US$120–140) at distributors.

4) Small complete hobby kits / Discovery Dish style feeds (LNA + feed + mount) — ~$200–$1,000 depending on kit

• What: “all-in-one” amateur radio-telescope kits (dish + L-band feed + electronics). These can be pricier than a stand-alone LNA but give a ready system.
• Example: Discovery Dish / 1.42 GHz feed projects and small educational dishes are sold as packages; prices vary but many beginner kits sit well under $2,000.

5) Bandpass / cavity filters (to protect the LNA from RFI) — ~$20 → several hundred

• What: inexpensive SAW / ceramic filters up to higher-Q cavity filters. A cavity or high-Q filter is often worth the money for suburban RFI rejection.
• Examples/prices: SAW / module bandpass filters for 1420 MHz appear for tens of dollars on Amazon/eBay; higher-end cavity filters or machined metal filters are commonly hundreds for better performance.

Practical takeaways / recommendation

• If your goal is the best performance under $2k for a backyard hydrogen-line receiver, the usual path is: buy a good pre-filtered radio-astronomy LNA (~$150–$400) + a high-Q bandpass/cavity filter (~$100–$400) and spend the remainder on a good feed and mounting. RadioAstronomySupplies and small makers on Tindie/GPIO are reliable hobby-oriented sources.
• If you want absolute best room-temperature performance under $2k, hunt the surplus/professional market (eBay, surplus dealers) for well-specified Narda/MITEQ modules — you can often find units in the few-hundred to low-thousand range depending on model and condition.