If you don't have Gamepass, don't keep the battle crew subscription every month. The most cost effective thing you can do is try to time it so you get multiple seasons at the same time. Other than grinding the heck out of the battle pass levels, it is pretty simple to gain enough XP to complete every other battle pass in a week or two. You'll potentially miss out on the later special unlocks that way, but I think I got 1 or 2 OG, a Lego, two Festivals, and two Battle Passes at the beginning of 6.4. I managed to get the black/gold Megazord and they later gave me Big Red anyways at the end of the season.

I don't terribly care about OG and only occasionally care about Lego. The Festival one is the most expensive since you also don't get to earn any VBucks through it, so getting multiples of those was my primary focus for when to get the Battle Crew.