According to [1], in 2140 each block will give 0.00000042 BTC (big guess based on mining trends). The reward per block goes down as the number of remaining coins reduces.

This introduces a very unstable system. If the price of bitcoin doesn't (double + increase with inflation), as the block reward shrinks, the rewards will become useless.

1: https://bitcoin.stackexchange.com/questions/10486/when-will-...