The numbers are the Influence currency (GW2 has many currencies) needed to purchase the buffs Influence is gained by playing PvE and PvP like doing quests, finishing personal storylines etc... but you can also exchange Coin (the main GW2 currency) for Influence. Now this is the interesting bit, Coin can also be exchanged for Gems which are bought with RMT in the GW2 cashsop and sold ingame for the Coin. So buy some gems in the shop, sell them in game for Coin, exchange the Coin for Influence and bingo buy your guild buffs. Perfect World have a similar system in their F2P games.