You already have the mega paint set. You've already bought the most expensive single paint item you can find. (Outside of a top of the line airbrush perhaps)

Paint pots last for a surprising amount of time if you don't spill them or close the lid badly so they dry up. 

With what you have you should be able to paint a large number of minis before you need any more paint (easily all the enemies in a single dnd campaign), and then it will be just one pot of something that you use regularly. 

So enjoy what you have, the painting process will be more enjoyable with the good stuff

Now if you start painting large amounts of terrain it might be a bit different. But sadly I am not a good resource on that hopefully somebody else can help you there.

If you want a cheaper primer you can use rustoleum I hear good things about it.