I was wondering what the best value I could get for an 8 speed cassette on my Trek 3900. How much should I spend? 50? 30? Is it possible to just buy the 7th and 8th sprockets? Those are the only worn out sprockets as far as I know (a brand new chain ran fine in every gear but 7th and 8th).

Have you looked at somthing like a SRAM PG-850, Nashar has these at $18.99. in 11-28 & 11-32.

Normally you can't get spare sprockets for cassettes, and even when you can find them, it will probably not be as cost effective as buying a complete cassette

I never really did take a good look at the SRAM cassettes or the $20 price range as I wasn't sure what the quality was on lower priced components, or whether there would be compatibility issues.. I'll definitely take a look though since It's a much cheaper option than I thought it would be.

The lower priced cassettes are generally just a bit heavier. They work perfectly fine though. You should be able to get a good Shimano or SRAM cassette for less than $20. Worn out cassette? Time for a new chain too.

Are you by chance abusing the granny chainring by using it with the 7th and 8th cassette cogs? This will cause rapid wear on all parts involved.

I actually have a new chain ready for when I get the cassette, and I never use the granny gears on either the cassette or the chainring. I stay in the 3-8 range on the cassette and don't go into 4 or below unless I'm in 2nd or 1st in front. I do use 6-8 a lot though.

I bought a 11 - 28 Shimano cassette for under $30.

Niagara Cycle Works has quite a few Shimano cassettes for well under $30: https://www.niagaracycle.com/index.ph...er_id=&sort=3a

I just bought one for $15.

With shipping you're at around the same price I paid, and I supported my LBS.

Love the SRAM 850 but a Shimano HG will serve you really well and perhaps a little longer... only difference between the price of cassettes is price as the tooth designs are consistent and performance is very even between them.