the cost of conversion to disc brakes really depends on what kit you use, how much work you can do yourself, and where you source your parts, and whether you are also converting to 5 lug in your case. if this is to be a daily driver or weekend cruiser, then just convert the fronts to disc brakes, and keep the four lug wheels, and you can do the job for around $500 total. scarebird has brackets you can buy that let you use later model calipers and rotors on your stock spindles. check them out at scarebird.com. there are other kits that let you keep your four lug wheels for between $700-1200 again depending on where you source the parts from.
converting to five lug on the other hand will drive the costs up as you need to swap out the rear end as well as the front spindles. and that will run you closer to $2000 total.
my mind is aglow with whirling transient nodes of thought careening through a cosmic vapor of invention.