I can confirm that Spirit's fares are cheaper at the airport. Whenever you get the email from Spirit that says "99% off airfare", it's off the base airfare which has, for me, always been between -$1.00 and $1.00. (Yes, negative airfares plus the fee nonsense... )

Going to the airport avoids one of those bogus fees - I forget what it's called, but it's about $17 per person per segment. I use Spirit about once per year to get to/from DTW, and my total ticket price usually ends up being about $16 each way - $10 that Spirit keeps and $6 that is legit tax.

Hey, it's the least painful way to get to Detroit without Delta status. Don't judge. It's usually four of us traveling, and I just walk over there to their counter before or after one of my regular trips on AA. Ends up being worth it to avoid $17 x 8 or whatever...

My 1 Allegiant flight to date was bought online. For some reason, I thought the online fee was lower but there was also some fee for not using "their" (or a specific) credit card. I could be wrong on that, but I remember thinking "this one isn't worth the hassle of booking at the airport." It's been a couple years though.