Time to put our math hats on! For now I'm putting my own guessing, if no one does, I'll start doing the math too...
It seems the price growth is not exponential (like in cookie clicker), but merely quadratic.
For example, at high levels the price of next digger update appears to be about 11,55*(n^2) where "n" is the current level. (There's probably linear and constant terms too, but it's hard to measure them precisely at high level, I'll check back on them after reset...)
The "All DPS +x%" skills add together. Combo bonus "DPS +x%" add together. Unit's own "+x% DPS" skills multiply together. To calculate the final DPS of a mustachio, the total (multiplication) of unit's own "+x% DPS" is multiplied by the total (sum) of "All DPS +x%" (the sum includes item bonuses), multiplied by the total (sum) combo bonus "DPS +x%", multiplied by temporary bonuses. Unknown: What about DPC?
Critical click is 3 times normal damage. The base chance seems to be 0. Unknown: How do the chance "+%"s compound with each other?
"+x% gold". It seems to be directly proportional to the damage done, regardless of whether the damage comes from DPS or DPC. Unknown: Is gold income affected by anything else?
The comments need to rounded up here... The most interesting research is in the comments.