Icex,
Adding a cab is probably doable but what KSSS is trying to say is the cost of adding the cab plus the original purchase price of the machine will be more expensive than just finding another machine with a cab.
Also that price sounds like a dealer list price. You could help yourself out some by checking around. I checked Machinery Trader and pulled this page for general information on your unit. I don't know how the formating will show up but the info is in here.
MachineryTrader.com
You Searched For: Manufacturer = CATERPILLAR; Model = 304; Year From = 2005; Year To = 2005; Country = USA; Listed Since = 8/29/2007
Auction Results Prices For Sale Prices
Total number of listings found: 7 10
Highest price found: US $29,000 US $44,500
Lowest price found: US $19,500 US $28,000
Average price: US $24,071 US $35,050
Average price w/out High & Low: US $24,000 US $34,750
Here is an at-a-glance view of current "quick sale", auction prices, and also current asking, or "retail", prices.
We have done computations on both "Equipment For Sale" & "Auction Results" listings for the type of equipment you specified. Above you see the average price found, the highest & lowest prices, and the average price computed again after throwing out the highest & lowest prices. For clarification, we've also displayed the number of listings on which the computations were performed. (NOTE: Items without a price have been omitted from the calculations.)