ThinkGeek offers the Star Trek Uniform Bathrobe in four styles for $39.99. Coupon code "1GBB" drops it to $34.99. Even with $5.49 for shipping, that's the lowest total price we could find by $40. This terry cloth robe is embroidered with rank and department insignia.

Update: It's now available only in Scotty (pictured); Kirk and Spock have since risen to $59.99 and Uhura is on backorder.