Rightlook.com and topoftheline.com sell them for $5000 - $10,000:
I think I can put a functional one together for about $2200.
This Haul-Master trailer sells for $350 at Harbor Freight:
Top of the Line sells 90 gallon water tanks with steel straps for $350 shipped:
Used 5000 watt generators sell on Craigslist for $300:
Used gas-powered air compressors sell on Craigslist for $400:
Used gas-powered pressure washers sell on Craigslist for $250:
50 foot pressure washer hoses sell for $180 at Harbor Freight:
50 foot air compressor hoses sell for $80 at Harbor Freight:
2 large toolboxes can be purchased at hardware stores for $100 each and bolted to the trailer:
Hardware (nuts, bolts, straps, etc.): $50
Total: $2160
Am I forgetting something?

I think I can put a functional one together for about $2200.
This Haul-Master trailer sells for $350 at Harbor Freight:

Top of the Line sells 90 gallon water tanks with steel straps for $350 shipped:

Used 5000 watt generators sell on Craigslist for $300:

Used gas-powered air compressors sell on Craigslist for $400:

Used gas-powered pressure washers sell on Craigslist for $250:

50 foot pressure washer hoses sell for $180 at Harbor Freight:

50 foot air compressor hoses sell for $80 at Harbor Freight:

2 large toolboxes can be purchased at hardware stores for $100 each and bolted to the trailer:

Hardware (nuts, bolts, straps, etc.): $50
Total: $2160
Am I forgetting something?