Actually, the point of the Gil Cost is a replacement for MP cost, not Tech Points. It's an MP cost swapping Secondary like Sacrifice and Charge. I think the costs should be cheaper, honestly. Otherwise you wouldn't be able to actually USE a Tech at First Level right out of the gate.
The formula should be something like:
Effective MP Level of the Tech Squared, multiplied by 15.
So, for example, a first level tech would cost 15 Gil per use, and an 8th Level would cost a whopping 960 Gil per use. Effective Level 0 MP cost (You know, the MP 1 cost scenario) should just cost 5 Gil per use. It may not seem like much, but remember, this is for EACH USE. That can add up.