$90 per gallon sounds awful cheap, the primer I use cost more than that. I would be leery of it's quality and how long it will last. The old saying of you get what you pay for comes in mind. I would use it if I was throwing together a quick driver but not a car that I care about or investing a lot of time and money into.
I also agree with the others in that this is not the time or area to take short cuts, you will regret it down the road when you have to start over.
If you are spraying a metallic paint, base/clear is the way to go. Single stage urethane is fine for a solid color because you can color sand and buff it with out issues. If you are on a budget then you can spray the trunk and engine compartment with single stage then spray the body with base/clear.
The single stage paint you're thinking of buying may be able to be color sanded and buffed but that is the paint, most paint can be color sanded and buffed once it is cured, you will still get blotching and discoloration when you are done. Metallic lays on the surface of the paint and when you color sand the paint you will cut into the metallic and cause it to look blotchy and if you try and sand out a run, the run will still appear even when the surface is smooth.
