Let me know if my math is wrong, but I think it's fairly simple to calculate the odds (assuming Funzio hasn't rigged it)
There are 6 items that can show up: (Car, armor, melee, Gun, $$, RP), so in each slot you have a 1/6 chance to get a certain item. To get the top items, you then need 6 of the same, which is simply 1/6 ^ 6. or 1 in 46,656. Since there are 6 top items, your chances are 6 in 46,656 (1 in 7,776) to get a top reward.
Getting 5 of the same items is 6 x's (1/6)^5 x's (5/6)^1, etc
Summary:
6-items - 1 in 7,776
5-items - 1 in 259
4-items - 1 in 52
3-items - 1 in 10
2-items - 1 in 2