I'm thinking about getting a Diawa aird costal reel 6:3.1 with 25 inches per turn and I'm checking to see if that would be ok for fishing crankbaits that are between 10-15 feet. I found one on sale for 65$ and it's hard finding 5:1 or 4:1 reels under 70$

I see no reason why that wouldn't work.

That'll probably work, or you could get a BPS Pro Qualifier in 5.2 or 4.7 online right now for $69. You can get them used PQ any day of the week for even less. I used one for deep cranking for a few years and never had any complaints. 

I just looked and they say they're on sale but when I check out the reel is still at 100$

I think the sale has ended on the BPS PQ. I scooped up three more of em at that price. Even without the sale its still a great reel. Best reel under $150 to me. I use it in a lower gear ratio for cranking and it works great

I would definitely jump on that deal... The Aird is great for the price, especially that price. I've used my roommate's a few times and was very impressed