greg calbi charges 4k$ per album (indie artist), and this is as best as it could be, within this money bracket you can find enginer who suits your music better,
for example, for hip hop you will aim for brian gardener rather the greg calbi,
this is not budget, and this is top of the league
as a budget I think that something below 500$ per album is something you could consider as budget mastering,
you still have a serious pro mastering engineer with proper studio, skills&experience, and hundreds of albums mastered by them
there's many here and around great guys doing great masters for 500$
then 300-400$ bracket per album is the shelf for engineers with less fancy studio and maybe bit smaller clients lists,
still providing great masters,
below 300$ per album (sth like 20$ per track) is the lowest budget mastering,
guys who are not very long 'in the game' mastering mostly using digital plugs (hardware is very expensive)
but still you could find a skilled engineers there,
then anything below that....well it's just my opinion, but I think if someone offers to hire himself for 15$ per track then he either is very desperate or/and is just making first steps in business...
I am not slashing first steppers, as everyone, even bob katz has to start somewhere,
I am just worried about thousands of 'mastering studios' providing services via net, with dodgy photos of equipment/studio.
just google mastering studio - apart obvious pros, you will get literally hundreds of 'cheap chaps' offering their service with 'them waves plugzz'

now, it's up to you who you going to hire to master your music,
it's a free world after all (or is it really ?....but it's a political matter, so I skip it)
peace