As for cost, it all depends on if it's worth it to you.
In my examples, I spent about $50.00 making a DIY 83802 & installing an IEC jack on a relatively inexpensive DVD player; about $20.00 ea. (x2) for a Volex & strain relief to replace the captive PC on some older mono-blocks.
On a personal note, it wouldn't bother me if I saw something advertised for sale & it had one of those adapter boxes on it but that prob. means the item would sell for less than a similar item w/o the mod. Something to consider if you ever decide to sell, unless it's already at the bottom of its price potential.