Question

How much does Helping Hand boost a move's base power?

Answer 1

It increases it by 50%.

Helping Hand is an increased priority move that will raise the base power of the adjacent ally's move in a Double Battle and Triple Battle by 50%.

http://bulbapedia.bulbagarden.net/wiki/Helping_Hand#Effect

Answer 2

Helping Hand boosts base power by 50%
Source