10GbE RJ45 networking is the most expensive, if you want to get into higher speed networking you need to use cards with SFP+ (SFPP) and QFSP+ ports, then you select either a transceiver for the application or use a DAC (direct attach cable) that includes the transceiver and cable in a single piece.

40GbE with cards and DAC cables are pretty cheap if you can get them shipped from the states. Cards are on the order of 15-50 (for both 10GbE to 40GbE QSFP).

If you are trying to figure this stuff out on your own from listings, you will be utterly lost. Use a big llm to help you out and cross reference, stuff can get tricky real quick.

If you want to get started, the lowest friction path is to get a couple intel 520, 540 or 710 nics and some patch cables. I started with just two 520s and a some cables for loop back.

A little bit more than the states, but not bad. https://www.ebay.com.au/itm/394258109292