Well for the daring and those with just $138 you can now chance a mod of your old GoPro Hero 3 and 3+ with the Kovacam GP01 Cage mod & 4 C mount lens package.
Right now on eBay you can score a Kovacam GP01 Cage Mod setup for $138 with the following:
Accessories and parts for modification works.
Metal Housing with bonus hotshoe adaptor.
screw drivers for modification works.
4 prime CS lenses.
C to CS mount adaptor
The Kovacam GP01 in action: