Costs update as of March 19, 2012:
Maxxis tire T135/70R18 listed for 149.00 with free shipping direct from Maxxis:
Goodyear T145/70R18 from Tire Rack quoted 189.00 plus shipping. Call in person 1-888-428-8355.
Bridgestone T145/70R18 from Tire Rack is quoted at 286.00
Tire Rack has back-ordered the T135/70R18
Rim 36-11-6-768-861 from GetBMWParts listed for 75.24 + 14.95 shipping + 3.95 handling:
Valve Stem 36-12-1-116-326 from GetBMWParts listed for 2.39 shipping with rim.
Hub centric ring 74.1-72.6 mm from 1010tires.com either 24.99 or 34.99 depending on plastic or aluminum free shipping (I found similar correct size rings on eBay for as little as 7.99 free shipping):
Total package price (cheapest options) 253.52 shipped.