Many variables at play. What caliber are you shooting? What distance do you want to spot the hits?
In general, I'm becoming a glass snob. But, if you're just trying to see some large caliber hits at 100 yds, an inexpensive one might work for under $100.
If you're trying to pick out 6/6.5/7mm sized holes at greater distances, some money will be involved. I use a cheapo Barska and can usually see .223 hits at 100 yds but it's pushing it.