Costs vary from research, location, shipping, and location.
My 2000 8.8 was $185 from the yard.
R&P(4.88s) was $199.
LockRight was $275.
Mad4x4 were $300.
Welding free (co-clubber).
Next, there are other small expenses - hard brake line with 'T', passenger side soft line, hard to soft line hold downs, parking brake adapter, rear ds flange (bolts to front yoke), ujoints (possibly), and brake hardware (calipers, rotors, pads).
Your TJ has a different brake line setup on the stock axle. You will need to change the drop line into a 'T' on the axle and add hard lines to the calipers (if your 8.8 has calipers (95-01)). The Explorer driver's side soft line is integrated into its hard line (can't separate). That is why you need an additional passenger side soft line. You are also going to need a bolt and nut welded to the tubing to hold the conversion point of hard to soft on the axle. The parking brake will also need an adapter to hold onto the 8.8 backing plate. The rear ds will require a yoke flange put on. Spicer has one that works with the stock rear ds joints. Might as well replace the joints since you will have the rear ds out. Then there is the maintenance of the brakes in the 8.8.
I figure I have ~$1300 into mine as a whole. I replaced the pads, 1 caliper (couldn't bleed the stock one), and the little incidentals added up. I still don't have a parking brake (not a problem as I have an auto).