IMHO, you will have difficulty to convince many designers and their clients to commit to $15 a month.
While I agree extra costs are undesirable, it is usually justifiable by your client provided they have the monthly sales to cover it.
At the end of the day, $15 a month is not much given the remarkably fast turn-around time FoxyCart can offer (mashed up w/ MODx, of course).
Some clients would be chomping-at-the-bit to pay the fee if it meant a fully functional, secure, scalable, template-able, SEO-friendly, seamless e-Commerce integration/solution in just a matter of hours, especially if the site's template is already complete!
The real fun comes with how you leverage your product data feeds and how creatively you integrate into the shopping experience. It really encourages thinking outside of the box, but does not require it. Regardless, your client (and their sales) will hopefully thank you for the recommendation, and the prompt return on their investment. Sure beats Flash-based shopping carts or managing separate CMS/templates
