Pep said:The cheapest is a DIY (do it yourself) mod. You can modify the swingarm yourself, but the rim will need to be widened to accommodate the 240 tire.
The least expensive way to go on the rim/tire is to send it to Kosman Specialties in California and have them do a "raw" finish. This will leave the welded bead in the rim (visible) but it's a clean looking weld. With shipping to and from Kosman you're looking at about $650. If you want the welds turned down smooth, add another $200.
You have a couple of options on the tire. If you can be content with the Dunlop 240, you can specify an 8" wide rim. If you feel you may want to go with the Metz 240, or possibly go to a 260 later (which will requireanother swingarm mod) you should go with 8-1/2" wide. The difference in price on the Dunlop 240 verses Metz 240 is about $70 with the Metz being the more expensive one.
Bottom line: Kosman modified wheel (raw) with Dunlop 240 and DIYswingarm will be approx $825.00
Do a search for DIY 240 or DIY Swingarm Mod to find details.