I think that you are understating the value here.
If you have access to a shop with all the tools and supplies (I do not) and spare time (I do not), then you may be correct in saying that it is pretty simple. However, I checked the price of the components, if purchased seperately and came up approx. $21worth of parts. Here is the list;
Posi-Lock connectors & Tap - $3.90
Posi-lock Fuse holder - $3.30
20A Relay - $4.00
Prewired relay connector - $3.00
2 x 5A fuses - $2.00
Wire, battery connects - $1.30
Shrink wrap tubing - $2.50
Zip ties (bag of 10) - $1.75
Note that the kit comes with Posi-Locks (expensive, but worth it), not just crappy old crimp connectors. Posi-locks are the best way to make a professional, permanent or temporary connection, in seconds (they can even be reused). The Posi-Tap allows for splicing into another wire in seconds (i.e. for power), without cutting the original wire.
Next, I still need to buy a the shrink wrap heat gun, find time to get all the components, assemble and test. Then I'm left with a bunch of spare parts, a heat gun that I'll probably seldom or never use again and a lost afternoon. For a one-time application that's a lot of expense, time and effort. I'ld rather be riding! The premade harness will take less than 5 minutes to install and will not look like a hack (a.k.a. ME) did the job. So, I would argue that the value for this type of product is very high for those who do not have access to a shop, lots of spare time or the technical know-how to make a top-notch professional looking wire harness.
As for the business opportunity, you bet there is one here! I've been researching heated grips and none of the vendors offer a descent all-in-one, plug-n-play type kit (heated grips, wire harness and heat troller). You have to buy all the parts seperately, sometimes from multiple vendors and wire the whole thing from scratch. I am positive that this turns many, like me, away. Not a smart business plan.
I figure that the total for the wire harness parts, purchased in bulk at wholesale, probably run $7 - $9, assembly labor and overhead for $1 or less. That's $8 - $10 for the finished product. Wholesale to big names that don't have them (i.e., FirstGear, WarmNSafe, etc.) for 30%-45% wholesale markup (market as plug-n-play kits), and on your business website for 100+% retail markup. Start-up costs would only be a few grand for parts, advertisement, website, incorporation (lega stuff), insurance, etc. Hire some High School kids part-time for big orders and you'ld probably do pretty good. I'ld buy one!