Great idea, I'd recommend allowing it to go through even with 0 help. Make it so the player needs 1200g in 'inventory' to be able to purchase the 1200 gold upgrade. Then have it put up pop'ups on everyone's screen. Then anyone who agrees throws in their share, and the original purchaser pays the remainder. So technically they could all say no, and he pays the full price.
The pop-up window could have two options
1. Pay your share (for 3 players, its 1/3.. 5 players its 1/5) or all your gold, if you have less than the share price
2. Don't contribute