Your question is a little complicated. First off lets go over some of the basics of cost breakdown for a web designer.
In-House Designer: no cost for overhead (because you supply that) no additional taxes (because you cover them) etc. ... basically a normal employee
expect to pay atleast $10-20 an hour
In-House Contract Designer: no cost for overhead BUT they only work on the basis of work needed (via a contract) ... most are also paying more for taxes + travel expenses ... only there when work is needed and paid the contracted hourly amount.
expect to pay atleast $20-30 an hour
Out-of-House Designer: overhead is covered by the designer (which means you pay a pro-rated amount per hour), they pay additional taxes, and can be either covered by a contracted pay fee (ussually less) or covered at a freelance rate (varies per assignment or job and is ussually more)
expect to pay atleast $15-25 an hour for a contracted designer and 20-50 an hour for a freelance designer.
Design Studio/Shop: Massive overhead, more than one employee, taxes galore, etc ... these are rated from job to job and you will be paying ALOT more ... good side: they take less time than a single freelancer, generally have alot better results, and you know that their price won't vary from job to job TOO much. bad side: expensive ... but you get what you pay for.
expect to pay atleast $50 an hour ... one client of mine was paying close to $600 an hour for design work i was doing for $45 an hour.
Also, anyone that seriously undercuts any hourly rate you see ... is either 1) inept at their job and can't afford to NOT get the job. 2) from india (not a bad thing, but alot of times there is a language+time issue so be prepared for that) and/or 3) you are hiring someone completely green to the field who may or may not know how to handle your needs. Overall, the phrase "you get what you pay for" is a good reference to this last part. Don't be afraid to pay a little more for superior results that will give you a better effect in the longrun, whether it be adaptability of design/code or plain old consumer effectiveness.