Thanks for the responses. Looks like the answer is: there is no answer, wait for the DMG.
Couple of other issues have been raised. First - which is a tangent to the original question but worth considering - is how much should a scroll cost. 100s of gold for a scroll is a LOT. If you can get a permanent +1 to your AC for a few hundred gold (by buying better armor), that seems like a lot for having just one scroll in your pocket. There's this idea that it costs 50g per spell level to copy a spell into your spellbook, but all that we know is that was written in reference to copying a spell from another spellbook. Nothing about scrolls was in there. It's reasonable to think that creating a scroll would cost the same as a "base" price: 50 x spell level. It's also not unreasonable to think that like most items, double that "base" cost would be the price of a scroll on the market. But these are just assumptions.
There's also the caveat that a scroll might cost 100g but then you'd have to pay 50g to get it into your spellbook - a sort of "double tax." That raises the true cost even higher. Which may or may not be unreasonable to you, depending on the style of campaign you want to play (see below).
Second, we have the idea of whether or not scrolls are even available to buy. And that's really up to each individual GM, honestly. Everyone has their opinion, and the DMG will no doubt have suggestions, but nothing is "right" here - only a preference. So no need to go down that path other than to state what can only be your opinion.