Update: In order to book this without joining the club just use the regular spirit airfare search for the dates that the site indicates are on sale!
Spirit is having one of their crazy sales now. In order to get the advertised price you have to join the $9 fare club (which costs $40/year) or have the Spirit credit card. However even if you don’t you only pay a $9 penalty each way to get in on the sale.
A quick search for a few of the cities on sale showed that it definitely is possible to still get the special fares in both directions.
HT: Yonah G.