You cannot finesse the system OP.
You can only get hit with an instant $150 charge on the spot. It. will. not. work.
The difference between Spirit and anyone else is that you think $150 is good until you pay another $100 on top of that for their extra fees. There will never ever be a discount to fly Spirit unless you go emptyhanded, no carryon. Like uhh, ship your bags via ups and bring your wallet and no carryon. The big difference, and why you think $25 is only $25 is your newb nature. They will gig you for checking the bag at the gate 4x more than doing it online.
Flying southwest or jetblue means not only free carryon, but also free checked bag.
Flying frontier, allegient and a couple other snowbird and seasonal carriers going to second tier airports part of the year are some regional cheapies. But, you can get similar deals on american or delta. I rarely see that united has my best price the past two years. If you are free to miss work, by all means gamble on Spirit, but know if they get a mechanical they will not want to put you in a hotel, or even bring in new equipment, or roll you into one of their 30 other flights that day like Delta. Flight is simply cancelled. Will that cost you more? Will you spend $15 on a bottled water and bagel in the terminal since you won't get snacks onboard? Will you arrive with a backache and leg ache, or experience a delay. Is that worth $20 to you?
Spirit is for novices who don't understand the system yet. They count on you being gullible. They pay their crew like shit too.