I use cheap guns for primer and save the better ones for topcoats. I have had really good luck with the "purple" gravity-feed guns from Harbor Freight. If you forget and the primer gets hard in the gun, you don't feel so bad about losing a $19 gun... I have also sprayed basecoats and clearcoats with these same guns occasionally and had decent results, but if you do a lot of painting a higher quality gun is worth the investment.

I bought six of the "purple" guns on sale once for $9.99 each. Of those six guns, one turned out to be a dud. It leaked paint around the flow rod and sprayed a lopsided pattern. The other five have been good ones...