Depends on how you approach post. If you're really serious about doing RED work, then a Rocket is totally worth the investment. From time to time they have used Rockets in the Battle-Tested section of the store too, for a bit of savings (didn't see any in there today, although you might check the FS/FT section of Reduser).
Otherwise one option to consider is converting the footage to another, easier-to-edit format before bringing it into your NLE (if you aren't sending out work to compositing/grading/3D types of houses or doing it in-house, where you'd need the extra resolution). To do that, you can build a render farm of cheap Hackintoshes and spread out Compressor across your network to chug through the workload for you. It's a bit messier than a standalone Rocket card, but you can save some cash and also use it for other things (rendering 3D etc.). Plus for some projects having an NLE native format really speeds things up when you just need to hack through things quickly.
I saw a Mini-ITX homebuilt cluster a few years ago:
http://www.mini-itx.com/projects/cluster/
Gigabyte has a nice hackintoshable Mini-ITX for $110 out now (GA-H67N-USB3-B3) that you could use - add an i5 or i7, 8 gigs of RAM (I bought my last 2 x 4GB DDR3 kit for $35 on sale), cheap power supply, and boot off a USB stick or cheap $35 hard drive. Gigabit switches are cheap and motherboards use Gigabit Ethernet connectors onboard now, so the network portion would be pretty quick. You could probably do an i5-based unit (read up on i7's actual Hyperthreading performance before purchasing...dunno if it's worth it for the extra $100) for under $400 a pop - with a $5,000 budget for a Rocket card, you could build 12 render units (48 processing cores).
Again it mostly depends on how you approach post. If you're going to spend some money and want to do a pre-conversion before editing, Aja sells a nifty unit called the Ki Pro for $4,000 that records directly to ProRes so you can edit in FCP right away. They also have a Mini version (Ki Pro Mini) that does 422 for $2,000.
