You will get asuma WB fragments slowly over time from survival trial and chase missions. I wouldn't invest a single coupon to get him, since u get him for free eventually. He isnt used very often as well, mainly 9tails/summon fights, any main blitz teams or coins trial. If you dont need iruka right away (hes good early but at your level you soon unlock kage treasure and with it better ninjas) then wait a little, you can get his frags for free as well from refining 5* ninjas.
If I were you i'd slowly invest into sage naruto, since hes the core of wind main (possible to piece him during 4-5 events with close to none coupon usage) and save coupons for power.
Regarding last sentence about your 5$- you are f2p, then your best friend is time. U can get a lot of stuff for free in this game by just waiting and slowly piecing together ninjas etc. I'd avoid using coupons on ANY "situational" ninja fragments, that wont stay in your team for next few months.