3"x30' (27k) is usually the norm, I've used 2" (18k) straps as well and they're fine unless you're really stuck (or loaded heavy). The strap in question looks to be a 2 - 3" but I guess its hard to tell from the pic. If they truly do have sewn loops on both ends and no hooks [very important!], then I'd say its worth it for $18. If it really is a 15k strap then it will either be smaller or cheaper quality. A true 4" strap should be rated for around 30k.
My only issue with a 4" strap is that it won't stretch at all but it'd be a great tow strap. LR
'09 Ford Ranger FX4 that gets abused a lot... but doesn't like to swim though.