You can buy rollcages (like Autopower or Safety Devices) or you could have a custom cage.
Also, need to consult the rule book about what the cage must be
1) Comliance for the club(s) you plan to run with. Its possible that SCCA and PCA have slightly different rules. Make sure you build something that can be used with whomever you run with
2) Material (steel, alloy etc)
2) wall thickness (based on material used and weight of car)
3) door bars (nascar bars) or not
4) rear cross bars ( "X" )
5) Paint/powdercoating
All of these can have an impact on cost. Delivering a car that is devoid of interior or will the shop need to remove your seats/dash etc. (in essence, the amount of prep work you do will lower your cost).
Example for a "off the shelf" cage:
Roll Cage Kit,911 Coupe up to 1989-Stable Energies