What price would you ask for to raise a level 3 base to level 4? 5?

It depends on what is still required to raise the base level.

CTP cost is usually just the straight ctp cost for upgrade.
Energy cost is anyywhere between 25-100:1 energy:ctp. Its a very wide range and depends on the seller. Of course, the higher the ratio, the better it is for you.
EM used to be cheap, but now its used for LM and valuable, so itll probably cost you around 125:1 EM:CTP for the remaining EM costs

After all this, there is the loyalty charge, where the seller loses loyalty to travel to your legion and upgrade your base. This can be free of cost, or up to several thousand CTP.