I hate to say that I normally sell my projects, but I suppose up until this point I have sold all of them but the Revolution, which I built for myself and is currently not for sale. Sorry.
That all aside I'd say $600-$1000usd will be the normal range of what you'll expect to pay if you were to commission someone. It will all depend on the parts they use and how much they feel their time is worth. I know the $1000 price tag I charge for a finished GC portable seems steep but you have to look at it from the right perspective. Firstly, parts alone for a GCp that includes batteries will range anywhere from $250-450 depending on what you buy. Secondly, you have to consider what the persons time is worth. If I spend 80+ hours of my free time working on a portable and I've incurred $300 worth of parts costs, you could essentially say I've been working for around $8.50 an hour. Which is about a quarter of what I make at my "day job" and only slightly over minimum wage here in the US. For the time invested, its not a huge incentive to keep churning out portables.
Just wanted to put that into perspective. Most people build these things for the love of it, not for financial gain. Everytime some flaxhead on youtube makes a stupid comment about the price I just want to scream.
Anyways, I'd say you could get a respectable modder from here or even the MBB forums to do it for a bit less if you really wanted to negotiate pricing. All of the people Hail listed above I'd trust 100% to get the job done and have it functional and looking nice, the only person Hail left out that I can think of that has also spent ALOT of time with the GC lately is Shockslayer, who I'd also trust to get it done.
Anyone else may be able to do the job but I'd ask around here as to how reputable/trustworthy said person is before you commit to anything. If you feel like waiting it out, I should be available in a few months.