Thanks for the answer, Chris. I think I have it now, but let me propose an example based on the team you listed, to be sure:
Prior to race 1, every contestant has $75M available to build their team (looks like paying to upgrade to a Pro Elite account increases that budget to $80M, but let's never mind that for now). From above, your team at this time is:
VER - $27.5M
ALO - $9M
DEV - $4M
MB - $20M
AM - $8.5M
AT - $6M
all of which puts you bang-on the $75M budget with no cash left over.
After race 1, the valuation of each asset can change, although the price you paid for them didn't change. So let's just say ALO does well in Bahrain and his stock rises, while none of your other assets change in value (for simplicity of the example).
If the market value of ALO goes up to $11M after race 1, you could hypothetically sell him from your team, and now you'd have the $0 left over from your original team budget + $11M for releasing ALO. That total of $11M is your available budget to sign another driver to complete your team. Conversely, if ALO's value went down to $7M and you chose to release him, you'd only have that $7M available for a different driver, and your team's market value would then be lower than the original budget.
And I see changes are limited after the second race, and making additional changes beyond that limit costs championship points.
I think I understand, and it sounds like a good system (other than the paid upgrades, but I understand the site needs to earn money somehow). But please let me know if I've got it wrong!