The missions for other characters pay a lot more than Gerald's missions - usually around $20k+ per mission. You used to have to be a certain rank before you could unlock them - not sure if this remains the case? You might be able to bookmark a mission after you have played it, which would then give you access to it.
We don't tend to do missions all that often, as most have a 4 person limit (although there are some for 6 people). So if we have more than that on, we tend to do more races, or, as in last night, just feck around killing each other). I'm always up for missions - probably more so than races, so am happy to join and do them too.
Some games like Snipers vs Stunters can pay well if you have enough people and do enough rounds of them - we haven't done them for a while so am sure we'll give them a go again.
But heists are the best way to make money. Levo, Stoney, Red and I did the Criminal Mastermind Challenge, which nets you $10m - which is where most of our current cash has come from. But if you do all the heists through in order with the same group of 4 people, then you get $2m in bonuses, plus all the money from each setup and heist. By far the easiest way to make money - and is also a good laugh. I would suggest finding 3 others from the thread here to do them with. Would be happy to be one of them
The performance upgrades tend to unlock organically after playing the game a while - you tend to accumulate wins. The other option is to repeat the same race over and over. I remember
@Damien and I did Down The Drain over and over one morning - the first race was a legitimate race to see who won, then the second race we made sure the other person won...and repeated that over and over. That way we still had an actual race for 50% of the time, but we both then unlocked all the performance upgrades. Each race only took a minute or so.