Watercolors from a new (not established) artist can go for about a sixty to two hundred pounds, depending on the size and quality of the work - generally to the lower end of that, and rarely below sixty from the paintings I have seen (this is, of course, framed and signed). Oils tend to be higher, around the £150 to £200 mark. For a while in the late nineties and *just* into the 00s there was a fairly large amount of numbered prints going for about £20-50, but I don't tend to look to closely at those. The last painting I bought was a beautiful watercolor of a bridge, and that was £120 from a woman who has never sold her work before.
Can't help with other forms (glass sculptures and such) as I really don't think about those much at all - and I have a habit of breaking things, so buying them makes no sense.