It depends on how picky you are.
You could probably make a reasonable facsimile out of "found parts" for $1-200, maybe less if you got lucky at thrift stores or already had stuff. Provided you're willing to alter and modify things.
Doing it yourself would be anywhere from $100 to $300+ depending on how picky you are and what/if you want props and how you go about making everything.
I'd guess you could find someone to do it for $200, but that'd be really cheap and I wouldn't expect much - I'd guess $700+, easily more with props for something close to the film (I'd expect to see someone charging a couple thousand for a really perfectly faithful replica).
There might be a "Halloween Bag Costume" one out there for cheap, but it will probably look as cheap as it is - but if it's just for a day of fun and not as a serious costume that's totally okay.
Like I said, depends on how picky you are.
You can do a "modded thrift store stuff" that people will recognise. Unless this guy is a HUGE fan or really into the idea and wants to spend a lot of money I'm not sure commissioning is the right path. Great if it is, just be prepared to spend more than $200 to even get something entry-level. Think of it this way - I'd charge $20 an hour (a pretty average rate) because I'm not sewing for minimum wage (if someone is, you get what you pay for or you're basically abusing someone's lack of self esteem) - and if I have to work 20 hours to make that costume, that's $400 in labour. Depending on how exactly everything is made it might take more than 20 hours.
Sorry if you already know that about the cost of commissions - but I didn't want you to be caught off guard and wanted you to understand where the cost is coming from. It's like paying a mechanic to fix your car - often labour is equal to or more than the parts themselves.
Decide on your priorities and what you care about. Maybe think about just having someone make the vest and wear a pair of pants with normal knee pads - that'll significantly lower cost and won't hurt the costume all that much unless he cares about being super accurate and doesn't mind spending the money to do so.
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan