A frag grenade costs something like $30 (and that's with military procurement overhead that would also raise the drone price to $300; mass produced third world grenades cost $5-$10 to their armies); a detonator, explosives and ballbearings for a DIY modification to that drone would be much cheaper.

Anti-tank shaped explosive charge would be hundreds of dollars but again, a fraction of the drone cost.

If the drone system is capable of aiming, you might want to try this wrinkle: https://en.wikipedia.org/wiki/Explosively_formed_penetrator