Now we could go ahead and develop a custom shopping cart but we're looking to save some time by just plugging in a robust existing cart and making the necessary modifications. We only need a cart solution, not a complete online store solution so the cart must be able to be used as a stand-alone application (we can modify our order page fields to be compatible with the cart.) We also do quite a few operations on our database (like inventory management and sales ROI logging) so we must be able to easily add our own subroutines to the code, so we prefer Perl which is what the rest of the site is written in.
I'd figure custom development of a high-end cart from scratch (with all the bells and whistles they want) would cost them around $6K-$10K. So anything below say $3K in cost (allowing up to $3K in customizations costs) would probably save time/money in the long run. I'm not going to go into detailed requirements as we are simply looking for the best designed, most fully featured product that can be plugged into an existing website and modified at any level.
HEllo if you are fluent in PHP and MySQl I would consider OScommerce; very highly config great admin a dream for a client I use it and when you get the idea about install you will be set up in a breeze if you really know your way there is also a way to turn it in a virtual mall but requires a few PHP manip of course we are speaking here of 100% free and open source