If you want to control your XM with stock HU - you have to shell out the cash. Otherwise I know a lot of people that bought generic XM kits and hooked them up to the stock HU - saved money, but lost a tad in comfort.
Actually XM kit that Ken sells on trdsparks.com comes with 1 year free subscription, so you are actually paying about $140 for the kit itself. Not bad of a deal if you ask me. (Check with Ken if the 1 year subscription offer is still valid, though).