FWIW, here's my rule of thumb for starting prices if you are selling (this gives you a little room for negotiation with your buyer):
3TE/25TE: lv1/max "popular" Limited Edition Series 4* (Armed Forces, Dark Elf, etc) or current series which gives performance bonus in the current event
2TE/20TE: lv1/max regular "popular" 4* (Gleb, Miriam, Partenope, etc.) or max regular "less popular" Limited Edition Series 4* (Dream Breakers...)
1TE/15TE: lv1/max regular "less popular" 4*
While I do feel that the level/price ratio is fairly linear, anything less than max level will give the buyer more room to negotiate, whereas if you are only selling max-leveled 4*, you as a seller are in a better position to stick to your original offering price.
Given that the TE supply is lower (exploits fixed and the introduction of TE(P) as event reward), but cost inflation is still high, you'll have to be patient to trade a 4* for TE as you wait out the inevitable flood of Luna-only offers.
As mentioned elsewhere, it may also be advantageous to max five decent 4* and trade them for a 5*. Once you snag your first 5*, it's much easier to trade up to a better 5* by offering your old 5* card and some TE to make up the difference.