It does depend on which wheels you have fitted and if you have good enough tyres to reuse.
If you have a common size wheel it's usually cheaper to buy a new rim and tyre then keep the old tyre as a spare, but if you have a less common rim they can be expensive enough that refurbing the old ones makes sense - if you can do some of the work yourself.
The cheapest I can find a bare Rim for my indespension trailer in 14" 5 stud on 6" PCD is @£50. Or with a tyre £100.
On the other hand smaller rims with the standard 4" (Mini/Imp) PCD are usually really cheap on line.
Take a look at TrailerTek - Good guys.