Unfortunately very hard to tell as the new prices have skyrocketed recently, and I haven't seen much going second hand recently. Also they vary quite a bit across the range, so for example (I'm assuming you mean a Historic) an R8 is usually quite a bit lower than an R9 (and you're usually just paying for a flamey-er top).

What exactly are you looking for?

I'd be very surprised if anything in decent shape would go for much below R25k, but it could go an awful lot higher depending on what it is. I may get shot for this but your best option is probably to ask the forum what the general feeling is about the asking price for any particular instrument when you find a seller.

Why get shot? Sounds reasonable.

Plain-top R8s have been pretty consistently the best deal in Gibson historics. But yeah. Over R20k. Probably by a fair bit.