cheap road bikes - some questions

So I'd resolved to do at least my first tri's on my trusty and much beloved mtb. I love my precious and nothing can replace it in my heart. Yadda, yadda. But this morning the SO says "I've been thinking...you really ought to get a road bike if you're going to do this tri thing...you look uncomfortable on that mtb after about 45 minutes." Yeah, I'm lucky.

Clearly, I can do the bike portion of a sprint on this bike. My goal for this year is a handful of sprints, but unless I just absolutely hate racing I really want to do an oly and 1/2 IM next year. I don't want to spend another $1200 - $1500 on a bike right now, esp one I'm not going to ride except on a trainer and in a tri...I don't like road biking. I am also quite a sucky cyclist - check the logs if you don't believe me. Working on that, but I don't think replacing the bike will help me, since those numbers are off a trainer right now and knobby tires matters not at all. OTOH, it's not fun sitting on the mtb trying to keep the cadence up for that long, and I don't know if that's me or the bike geometry, both, neither, or what.

I'm obviously not competitive and will be happy to finish anything other than DFL this first season. If you had, say, $600 to spend, knowing you had a good solid mtb that you could put slicks on, would you bother to buy a really cheap road bike? Or would you wait and see if you were hooked, and then put that money into a good bike? Are there any road bikes out there that are even reasonably decent for less than $800? What about in the $400 - $500 range? Would rather not go with a used bike only b/c the LBS gives lifetime free tuneups and a lot of other perks on bikes bought in their shop - pays for itself pretty quickly.

I'll go talk to my LBS about this but wanted input before they start showing me things I can't/shouldn't afford...they're really good at selling me stuff.