I would say check into a third edition if you want to go the cheapest route for The Gunslinger. That should go about $90+. A second edition will be somewhat expensive at $200+ and a first edition will run you $500+ I would think.
If you want a copy of Drawing of the Three first edition signed by the artist, Donald M. Grant Publishing has them for $75.
As always, check sources like ABEbooks.com and ebay before you buy as you can sometimes find bargains. Beware of something that looks like a real steal. It often is a steal, but it's the seller doing the stealing.