Dear Poster,
I saw that once you already accepted yanoe's bid.. What was the problem with that?
Also, your expectances need to be narrowed.
I understand if you have enough of "xx years experience, we will do .... unparalleled satisfaction"..
Still.. I have written rootkit detector, and trojan finder. But I won't post nothing here until you clarify.
Writing the utility, as others said, even with realtime access shield, *may* fit in your price margin.
But no one will bring you a state-of-the-art (e.g. up-to-date) virus signature database for that price to you. And even less will commit to regularly update it. It is a 24/7 task for a complete team just to "keep up" with new threats.
I can however offer consulting on the matter as well (like methods on how to "ease" the signature keeping, running an automated lab, or developing a "free" utility helping to automatically get the latest of suspicious for latter inclusion.