They have transceivers for $8 sure, but are they 1/10Gbit transceivers, or are they 1/2.5/5/10 transceivers? Will they work with your switch? How long will they work for? (SFPs get crazy hot.)
Mikrotik makes an SFP module that's super reliable and supports 1/2.5/5/10. It's $53/unit.
Why would you want to run 2.5G or 5G over fiber? 10G fiber transceivers are dirt cheap and barely use any power. It's the 10G copper transceivers that have thermal/power issues...
I had 10G fiber transceivers crap out on me, for what it's worth. The Mikrotik SFPs are quite good though.