What a (good) advisor will do is as follows:
1) talk to you about your circumstances and ascertain all the options for you to come over to the UK
2) when this has been decided, you may then wish to instruct them to make the application on your behalf, which means completing the application forms, assessing supporting documentation, writing any necessary arguments which may be needed, liaising with the Home Office/ECO to iron out any complications, and generally making sure your application has the greatest possible chance of success
3) advising you on any future immigration issues which may arise, such as eligibility for Indefinite Leave to Remain, bringing in dependents etc.
As for how much we cost? Well, for that you have shop around. Some charge per hour, some charge flat fees, some it depends on what the case is, but all must be transparent about what you are paying for, and all have had to take lengthy exams to prove that we know what we are talking about.
And no, you can't come as a visitor then switch to either a work permit or a spousal visa...though if you find a suitable job, husband within the six months you can always come back to the US to apply. You certainly wouldn't get nationality for quite some time.
It is all a bit more complex than I think you realise.
Hope that helps.
Vicky