I’ve worked through the milestones and the unit cost of the crystals to come up with how many milestones you can achieve in both Alpha and Omega, assuming you only buy the better (300 unit) crystals, and spend all units earned in the Omega track on crystals, given a particular level of spend including zero (F2P).

This presumes most spenders will be targeting milestones in the Omega track and not the Alpha track specifically, although if you want to figure out how many units it will take to reach a particular Alpha milestones it is not hard to interpolate. At zero units spent out of pocket (but spending the units in Omega milestones a player can reach 78 points in Alpha and 44 in Omega. It takes 36900 units to completely max out Omega. (Note: 185 points is the maximum in the Alpha track).
The reason why the units jump around is because it takes a certain number of crystals to reach the next milestone, and sometimes you have help (units or crystals in intervening milestones) and sometimes you don’t and have to spend more to get all the points necessary to reach the next milestone.