A 'penny' is (was) a very small coin worth almost nothing. You could buy little to nothing with it.
'for thrown small pennies' is an expression derived from this. It means to 'throw' or invest only a little money at something. It essentially means if you only assign/invest/give a tiny amount of money at something, then you should expect only very little in return. It can be used literally (you only physically spend a tiny amount of money and get little in return) or sometimes metaphoricaly (Rather than little money, you are investing little effort or time and the result is negligible as well)
(1) I only spent a small amount of money on renovating my house. I can expect little (in return) for the pennies thrown'. I am saying I spent very little, therefore I should expect little in return.
(2) "I hardly studied for the exam so for the pennies thrown, my failure was expected."
I put little to no effort into studying, so my failure was only to be expected.