I would guess you can get a rifled barrel for under 250. Haven't priced them so hard to say. I would recommend the barrel with integral cantilevered scope mount, if you plan to scope the gun. If not, the version with iron sights will work fine.
As noted above, you may be able to get acceptable accuracy with rifled slugs in your current barrel, particularly if you don't plan to shoot that far. Another option is to install a rifled choke tube, and some sort of sights on your current gun. Sights are made that will attach to the vent rib on the barrel (if you have one on your barrel). These have been known to perform well with sabot slugs in the lower velocity ranges, say under 1700 fps.
Obvioulsy, using rifled slugs in your current setup is the cheapest option, if you can get them to shoot well enough. They'll do a number on the deer too.