I'll be ordering, soon, never fear.
There is a "preceived value" issue, where your software is SO below the competition that it might be perceived as having a lower value...not just the price, but the useability, etc.
Maybe you can inject a bit more "B" into the site with a quick explanation of why it is so low-priced? Nothing too fancy, just write from your heart. ("I am sick and tired of paying a lot for a program and having it turn out to be..." ... oh wait...that's MY heart!
You are right in that Excel is exceedingly simple to hack, but once someone purchases a copy, that's not much of an issue. Their copy doesn't need to be hacked, and can be distributed with ease. So I wouldn't worry too much about protecting the files in their useable form. You are in a position of being forced to trust people, and you probably won't get burned too badly.
If you want to time-limit or feature-disable them, you'll need a more robust format anyway.
You might consider publishing a few of the lesser spreadsheets as "shareware", where you heartily suggest that folks who use the software send you a bit of something in return. You'll gain users, and sales of the whole suite may compensate. Plus, you'll enjoy good karma, and gain a loyal following of people who look forward to your future works.