I think this should depend a lot on all sorts of factors, including local availability, the bargaining skills of the magi, the mood of the day, and so on ...
In our most long-running saga, the covenant generally allows the members to train one-for-one with covenant stocks, unless there is a shortage of the particular art, which is very likely to happen for Creo, Corpus, and Vim. In addition to that, we played a session at the Midsummer Fair in the Greater Alps. At that occasion, I let the prices vary according to the scarcity stipulated in the 4ed book. The most expensive art was Creo, at 15p Vim to 4p Creo. The cheapest were Vim, Auram, and Terram at 1:1. The most expensive form was Corpus at 10p Vim to 4p Corpus.
I think the double price for techniques stipulates that technique vis is a lot scarcer, and also that books are scarce enough to make vis studies commonplace. If the availability of each art in vis sources matches the demand, I do not think that relative price is sustainable. If an art is sufficiently scarce, the price makes sense.
For realism, I would have liked to see a lot more variation. I imagine aquam sources being as available as any other form, but with significantly lower demand than any other art. Corpus, I would think, would be scarcer than most forms if you do not want to massacre faeries, and in extremely high demand for healing and longevity. OTOH, for playability, we have to keep it simple, and the Mercere rates do that at least.
As a player I agree with you. Technique vis is twice as useful for enchantment, so if the right form vis is available, the technique vis is saved. However, when doing, say, a ReTe enchantment, I can just as well use the Rego vis I have than calling a redcap to trade it for the same amount of Terram.