Welcome to the site!
If you want to make a very, very clean weekend driver, I'd expect to pay around $20,000 in total. I purchased my '73 several years back at the age of 17 (now 22) and have around $10,000 into it as it sits. Figure in another $5,000 for engine/transmission work, about another $3,000-3,500 into interior parts, and another $4,000 (at a discount) for the final prep/paint and I'll have a very nice, clean car for ~$22,000. The best advice I can give you from experiencing it first hand is to take your time and do things right the first time, rather than slap everything together quickly and have to put more time and effort into fixes later on.
Best of luck with your E-body search and as already mentioned, good purchases can be found. I found mine listed in a local paper for $2,000 and ended up purchasing it for $1,750 after seeing it.