Your question is answerable with a yes or no, so I'm going to have to say no.
The main reasons something like this doesn't really exist are likely as follows:
- As with some sets in LEGO, the prices for (say, Halo) are subjective and diasagreed upon.
- MegaBloks is fairly new, compared to LEGO, which is why you also don't see a pick-a-brick for KREO, Nanoblocks, etc.
- Additional, "loose" bricks aren't manfufacured specifically for this purpose.
- Perhaps most importantly, the range of set-specific pieces is far larger than a lot of other manufacturers, thus each piece's cost is based on which set it's from.
But will we see MB pick-a-brick in the future? Probably; stay tuned.