You have to start reading about the different systems out there. LOTS of stuff on the internet and YOUTUBE.
You could buy the FAST EZ-EFI kit for $1850, Throw in a $110 fuel pump, a roll of 3/8 brake line from NAPA, a couple of fuel filters, modify your sending unit. Your could be in it for less than $2K.
I've picked up 2 used Holley commander Throttle body systems on craigslist for $500. Older system that works very well.
I've found A Holley 4BBL throttle body at a swap meet for $15, that I'm going to run with the FAST EZ-EFI multipoint conversion kit $870. You could use a GM 454 TBI throttle body and sensors from a junk yard, add a 4 bbl adapter plate, plumb your $110 pump, add a relay and a couple of sensors, then your system is self tuning.
Many ways to do this cheaply but you have to be inventive or have someone help you with the first one. Check out the youtube videos for MSD's ATOMIC EFI and HotRod Magazine's FAST EZ-EFI. Looks easy enough to install. You just have to dive in.
Self tuning systems are the way to go now. Check out this video from Holley:
http://www.holleyinjection.com/instructional-videos/self-tuning-capability Most new systems are self tuning. If you want to do this TOO cheaply, stay with a carb. Simple and easy to fix on the side of the road. For my long POWER TOUR road trips, I already bought spare parts in case something fails. Spare EZ-EFI computer $190. Spare distributor for my Edelbrock EFI $400, fuel pump, etc. With a carb, you need a can of gumout and a screwdriver.