Diving wetsuits generally are full sleeves. While diving, generally your legs do most of the work (swim fins provide plenty of propulsion, and your arms mostly hang loose, doing very minor work). Kayaking is just the reverse. Your legs are mostly motionless (yes, plenty of push/pull controlling the boat with knees, hips, feet, but no real motion, straining against the wetsuit), while your arms are almost constantly in motion. Thus, you will spend a lot of energy straining against the restriction of the sleeves of the (diving) wetsuit. For this reason, virtually all paddling wetsuits are farmer-john style (no sleeves), to allow full freedom of motion. So, yes, it does seem odd that you may be able to get diving wetsuits (with thicker neoprene and more neoprene) for less than paddling wetsuits. I can't excuse or explain the reasons for that. But there are reasons you would not want a full, thick, diving wetsuit for paddling.