I love to fly F9 because of the low costs, and I've done a lot of day trips and spur-of-the-moment kind of trips for < $100 r/t. I've even planned maybe a month or so in advance and had flights cost around $120 r/t. This is kind of what I've come to expect from F9.

So, I decided I want to do a long weekend in Milwaukee in August. When I looked at flights yesterday, MCO-MKE in August, both flights on Saturday, came in at around $1,299 base price WITH Discount Den. I checked again this morning and the same route on the same day is $998 r/t. Is this some kind of weird error? Is something going on with pricing? Why on Earth would someone pay as much to fly F9 to Milwaukee as they could pay to fly to Paris on B6? Something seems... wonky.

Are you looking at non-stop flights? If so, you are probably looking too far out. About 30 days to six weeks is when the best fares seem to appear. I get the impression that ULCC passengers book later than your average airline customer. Either that or this is a prime travel period and Frontier thinks they can get people to pay the fare they are publishing. I would bet fares will lower as the travel date gets closer unless travel demand really is strong this summer. Either that or Frontier will cancel the flight

I bet you're right. It's really rare that I plan that far in advance; I'm usually a spur-of-the-moment kind of guy and just a few weeks is a long lead time for me. I'll check again in July and see if the fares are more... uhm... Frontier-like.

I wonder if this is typical of all ULCCs? Or just a F9 strategy?

I've noticed the same trend on Spirit and to a lesser extent on Allegiant, too. Although I've never seen fares this high in absolute terms. Do a search in calendar mode so you can view fares for a month at a time and you can pretty easily pick out when the discount fares end and the place holder fares begin.

At that price range, did you at least looked at other carriers to compare? If I'm set on buying the tickets right now, that's what I'll do. Per google flights, I'm seeing those $900++ fares for F9, but they are around $400 on others.

It's been pretty common for summer flights. We may not get back to those days where you could do round trips for under $100 even in the fall.