Currently an M67 frag grenade costs the government about $60 in quantity of 1500, while a 5.56mm bullet costs a commercial buyer about 35 cents in quantity of 1000. So if a 35 cent bullet costs 2 , a $60 grenade would cost about 350 .
Unlike bullets, you can't buy frag grenades at wallmart, so there tends to be a rather large markup from people who sell them. SR also tends to make it far too easy to find people who are not trying to set you up or rip you off when you go looking heavy firepower.