Well, there are the factory customs. Some are no longer made, but can be found on Gunbroker.
The Elite: very limited run, the slides flats polished to a high luster and the inside of the grooves dark. The other enhancements, others will have to point out. It is a very striking looking gun.
Special Forces: another very limited run, sent to Wilson combat to be finished in their Armor Tuff finish and certain parts "tuned" by Wilson or factory? There is one on GB now for 5k, one member tried to sell one here. His price bottom out to $2500? I believe, I don't know if he had any takers.
Robar - factory tuned parts and gun sent to Robar for NP3 self lubricating hard finish. A gun well worth the money. Limited Run. Used ones go for around 2k.
Current Customs-
Factory - Tribute- cosmetically enhanced to resemble early model Farmie. cost?
?--Desert tan duracoat finish and brown grips. Name and cost?
New Larger sights on slide - availabilty yet? cost?
Covert: Special Forces wannabe done by an Austin, TX Rohrbaugh dealer. $1700
Privite Gunsmithing - you're on you own. Bob Coogan's name comes up the most on this site. His work has included porting, checkering, stippling, sight enhancement.
Maybe owners of Bob's work will show pics and speak of costs.
Good Luck