I recently bought a Surly World Troller frame. Overall cost (including case, etc.) was a maybe $100 more versus adding couplers to my existing Troll frame. Ended up with basically a free frame in the process, which has since been repurposed as a 1x11 commuter/trail bike. Went with Bilenky because they offered the best price as a complete package. I would only consider retrofitting on a really nice frame that is in very good condition.