You wouldn't typically count the registration cost in the acquisition cost, because it varies (depending on how long you hold the domain, whether you have a discount from GoDaddy (i.e: many experienced auctions participants will be paying ~$11 instead of $21)).
Personally I think there is very little value in 3 character .com domains just based on their length. You can pick them up cheap quite often, certainly better options than 3e2.com available in that price bracket. Numbers in domains are too ambiguous which rules them out for most use cases (because they fail the radio test).
(You could probably flip a short domain like this on Namepros or equivalent venue by taking advantage of the misconception that CCC.com is valuable in the same way LLL.com is valuable).
There is an exception, numeronyms (specifically numerical contractions): you might recognise i18n (internationalization) and a16z (the VC firm Andreesen Horowitz). Nerds love numeronyms and so if you can secure a great example that fits a technology audience, there could be potential for it. My name is a numeronym.
b0w.com and f3t.com both sold at a GoDaddy auction for a similar price recently, and I think those have some potential. I was in those auctions but missed the end sadly.