If I'm not mistaken the % chance of getting GF was lowered to 0.5% recently.

Thank you for pointing that out to me. I went to check, and on November 4, the probability was lowered.

do you think the glint spend per season is compensated byt the extra won SPS?

It depends on the league you play in and your SPS earnings per battle won.
For example, if I hire a summoner and spend 50 DEC per day, I only need to win 2 more battles per day thanks to that summoner to make a profit.
Before hiring a summoner, I always do a quick check:

• I look at my last SPS token earnings in my last battle won
• if the cost of a summoner is easily covered by a battle victory, that's the first positive sign to decide to hire them ;)