The visa itself isn't more expensive, but the cost of the solicitor would be pretty high. You can do the application by yourself if you like, but since it's outside the rules going it alone is, IMO, a scary prospect. Plus, a solicitor could clarify your situation with regard to the NHS. UKBA takes a very dim view of people getting free care when they're not on a visa that allows it, and as I said before, I'm not sure what the implications of an overstay are for NHS, but my guess is that you wouldn't be eligible. You'd probably be OK going without medical treatment for a few months if you weren't expecting, but obviously you need prenatal care, and that would cost a LOT out-of-pocket. So if you consider solicitor fees, medical bills, the FLR(M) fee, and months of waiting in limbo for a decision, that works out to a lot of money and stress. Whereas, if you go back to the US, apply for the spousal visa, and get the priority service ($300), you could have the whole thing dealt with in less than a month. It is a lot of money, but as I said before, there's no inexpensive option, and at least it would be quick.
Take comfort from the fact that it is very unlikely that the overstay will result in a visa refusal. You'll get this sorted eventually. In the meantime, though, you've got some tough decisions to make. Good luck.
ETA: One other thing. If you decide to apply for an out-of-time FLR(M) without a solicitor, then we (the forum) would not be able to be of much use to you, since we're not qualified to answer questions about applications that are outside the rules. All we can really say is what's in the UKBA guidance and what's worked in our experience, but our experience wouldn't be relevant to your situation. Just something else to think about.
