Maybe driving: if you're driving anything larger than a cargo van, a sleeper is a legal requirement, because you must log time spent in the sleeper berth, or produce motel receipts upon demand by the authorities, to prove that you spent the legally required time resting in a legally approved manner. In a cargo van, the sleeper can be as basic as a sleeping bag on the floor, if you choose.
I'll leave the cost/benefit figures to the guys who are good at that, like LDB, but sleeping in a motel is not economical, or convenient, either. Like truck stops, motels don't appear exactly where you would like to park, all the time, and even if there is one, what if it's "no vacancy"?
If you can't afford the sleeper, how will you pay for a motel room almost every night, even if you find one?