Web Services are not ready, yet. From what I understand only a few elements are available.
Yes, ROI has to configure the mapping between MAS90 and your site based on your specs, but once it's in place ROI is virtually out of the picture. They've already integrated with most popular storefronts, so most of the time they do not need to reinvent the wheel. It works like eBusiness (without the approval process), as a MAS90 module. No other software is required. Most of our clients that use it never need to go to the storefront for day to day stuff.
And, yes, In Synch will synchronize your current customer pricing or any other pricing currently in play in MAS90. It has all the same touchpoints as eBusiness and adds the freedom to use almost any hosted site XML , SQL, etc. You can create UDF fields to hold web specific fields. You can model these after eBusiness.
True, epicor and netsuite 'just work,' provided you like their look and feel, and won't need additional functionality that's not available.
Separating the two allows a client the freedom to pick (or change) a storefront that fits its needs without worrying about compatibility.
For example, we have a specialized wine merchant that uses In Synch with Nexternal. Nexternal's stores specialize to the wine industry which has special rules about shipping and allows for wine clubs, etc. Another is an organic dog food manufacturer that sells retail and wholesale. They use websitepipline.
Either way, any solution will require some work up front both in design and functionality. The value is in the ongoing maintenance and fulfillment of the store.
If you're interested in the cost, the module is $4,995. They estimate that most jobs can be completed in 24 hours @$200/hr, so they include a fixed-fee $4,800 not-to-exceed budget. Any additional ad-hoc programming is additional. Annual Maintenance is $550.