Couple things... It depends on the other equipment you have. Also play style comes into play as a PvP player, NPC player and Missions player will all have varying amounts of repair costs. Are you Builder? What level planet structures do you have? And wait until you get past 160 so you can draw Aidonium.

Personally, I wouldn't even bother with medium weapons. Just go straight for the heavy version. What I would do in your place would be determine how much PROFIT you make every day after your upkeep is factored out. Then figure out the CHANGE in upkeep going from your current weapons to Heavy Thetas (http://galaxylegion.com/wiki/index.php/Weapons). Will your daily PROFIT cover the increased upkeep? If not, how close is it?

a good 5k mining should do it.

5k - hmmm i might need to buy some mining planets then

Doesn't matter. They aren't worth it at your rank.
if you can afford them you are either lacking elsewhere, or have something/someome supporting you (gp, leftovers from a reset, gracious benefactor)
The mere fact that you asked tells me the latter is not your situation and with that it also means you should not be upgrading to thetas period.
Also should note that switching to builder so as to afford them would be a horrible idea.

Personally I would just stick with the light theta crons. They give a large Atk jump from nulls due to a tier jump in tech from nulls to thetas, but still have an affordable upkeep