The range on these things is about $500 to $3000 depending on the model, year, and condition. The good news is these bikes are worthly of a restoration. Just keep in mind a restoration like this is first and foremost a labor of love. Someday you might be able to sell it for the cost of the new parts, powdercoating, etc. but don't expect to recover your labor it took to get it into shape.
I've been involved in BMW R50/2, R60/2, and an R27 restorations over the past two years. By the time you get thru the second one you have the process and the parts resources figured out. If you can find a good source who has done your model before it really helps. So does having a "go by", a similar bike together, to follow as a guide during reassembly.
I'm about to tear into a '72 Mick Andrews Replica Ossa. It is already pretty far along but needs the frame painted the correct color. The body work is done (correctly) and the controls are all modern. Wish me luck.