Well, it all depends on your mechanical ability and fabrication ability. You can save a ton of money if you do the work yourself and have the equipment to do it correctly.
The adaptor is advertised at $339 on Marlins website, and figure another $100 for a top shift TCase from a local wrecking yard. If you can do the install yourself, then you're good to go from there. If you can weld, you can lengthen your front driveshaft yourself using your old rear driveshaft tubing. (the front shaft won't need to be balanced unless you use 4wd at high speeds frequently) The rear driveshaft will need to be shortened, and I'd reccomend getting a double-cardan joint in the rear driveline. You can use a late model Tacoma 2 piece driveshaft and just use the rear of the 2piece shaft and have it re-tubed to the proper length. The Taco shafts are readily available at junkyards ($150?) and have the correct flanges on them to bolt up. That's how I ended up doing mine.... Figure $100-150 for re-tubing the rear driveshaft and balancing, if everything else is ok with the shaft. You'll also need to clock your rear axle so the pinion points at the tcase output at the correct angle. Some get away with shims, some need to cut the perches off and weld new ones on so the axle is at the proper angle- it just depends on your setup.
Plan on clearancing the horse collar crossmember in the frame so the tcase output doesn't hit it, and I'd pony up for the dual case crossmember from Marlin ($195) if you can afford it. The extra mount on that crossmember really reduces drivetrain flex and reduces the stress on the single mount that results from using the stock crossmember.
All said and done? $950 or so, if you can do the work yourself. It CAN be done cheaper, but not necessarily BETTER.
