around an eighth is ideal for a first time experience, if the mushrooms are dried psilocybe cubensis (or a species of roughly the same potency) many people would suggest half that amount, though. my guess is that you haven't tripped before. if you haven't, you might want to just take a half dose to get a taste of it before making the plunge. psychedelics don't just get you "high," it can be a very novel and alien experience, and some people get very uncomfortable as a result.
first time I'd say take about 2 grams or so. around here it's about $10 a gram, so your looking at $35 an eighth on average, but occasionally you'll see it for $30 if mushrooms are in seasons or $40 in dry times.
jesus people....an eighth is too expensive even at $25, but at least thats a respectable, though inflated, market price. i've paid 30 bucks for an eighth once, but anything beyond $20 is too much, though 25 is common enough to settle for it. but 30+ dollars is highway fucking robbery, no matter how good they are. sorry for your luck...look for new suppliers, or grow your own. you can do it small batch just for yourself...
I used to get 30 buck ounces, highest I have payed was 30 for an 8th of azures. For 50 bucks you can score a cheap pc, jars, birdseed and a syringe.