I built all my amps on less than $100 budgets, most nil. You have to look for guys with old junk in their basements that's rusting away and ask nicely for the old tube junk. I have a reasonable tube collection that I didn't pay a dime for. Iron isn't as easy to come by and also is the most significant part affecting sound so you might want to pay for some Hammond transformers. Although I made a really nice sounding midrange amplifier out of an AA5 radio (All-American Five, alluding to the 5 tube design), although it's unsafe as heck. My 5W 6L6 SEP amp (Revision 3) works great and I use it every night or so for bedside listenin'.
For everything else I listen to Frankenhouse and Hept'AU7, both PP amps of 10-15 and 7W respectively. Hept'AU7 is a PPPP (i.e. highly paralleled) triode amp and Frank is pentode 6V6.
I would recommend a cheap triode like... pfff... 6V6 and 6L6 have little triode-mode power so maybe a sweep tube. Those can't be too pricey, right?........nevermind. Well then a 6L6 triode or UL amp will sound great for starters, though low on power. After experience with that you might try pentode mode and decide it sucks, because you need negative feedback. If you have an o-scope and sig gen and know how to use them, you can adjust NFB properly and then enjoy the sound as I do at night.
PP amps are a bit more complicated so get some experience first. Once you've done the above or so, you can build a bog standard 6V6 PP amp, then if you think you need yet more power
a 6L6 PP. Money is proportional to the square of the power, things get expensive quickly. If you have a lot of scrounged parts and such, it can get pretty cheap. There's absolutely no reason good quality audio need be for the rich only, it's open to the resourceful as well. Until you get into cables and "vibration damping cones" and colored Sharpie markers on your CD edges..... at that point, it's the spending of money that improves the sound, not whatever the money buys. And BTW, don't listen to reviews or opinions because 1. they're probably selling something, 2. nobody hears things the same, 3. most of these people believe
in the sound (audio religion you could say). Hence the self-photo-in-fridge sound improvement techniques also.