Druid Shapeshifting: Alternate Form or Spell?

[shadedmagus]

I'm trying to come up with a simplified way of implementing Alternate Forms as character advancement for druids in DFRPG, since Kromm introduced one type in the "Beast Flight" druid ability. But I'm having trouble with this aspect of it:

Basic Set, Alternate Forms, p. B84:

If you have multiple forms, pay full cost for the most expensive form. The less powerful Alternate Forms cost a flat 15 points apiece. Minimum cost
per form is still 15 points.

DF 5, Becoming the Beast, p. 8:

In particular, remember that a druid who knows multiple forms only pays full price for the most expensive one; all remaining forms are a flat 14 points apiece.

(I understand that the power modifier accounts for the change in point cost.)

The thought I keep coming back to is to have the druid pay most of the point cost up front for the ability to take Alternate Forms, so that all the actual forms only cost 14 points. But I'm having a hard time coming up with a cost that would cover the wide range presented in DF 5 (between 70 points for the Wolverine form and 183 points for the Bear and Great Cat forms, with most of the others coming in around 127).

Is this a good way to approach this? Or should I just have them use the Shapeshifting spell for the more expensive forms and keep Alternate Form for smaller creatures?