If you're gonna buy a ring for a lady, you should know damn well what the lady wants. If you don't, you shouldn't be buying the ring. Subtle conversations are a good thing to have if you want it to be a semi-surprise.
Practically speaking, every girl wants something different. Some want a big diamond (mined, created, or no preference), others a big nice rock (eg, sapphire/ruby or emerald), others different rocks. Some want a brand new ring, some vintage, some don't care. Some want little rocks, some no rocks, some don't give a fuck, some want a $20 ring and a vacation, some like their hair pulled and some don't... you get the idea, yes?
There are lots of ways to go frugal without being cheap, but they depend on the lady in question.