If it would cost that much (10 m+) it should have some preeeeety good rewards. Some suggestions :
Private hunting zone that changes pokemon type everyday where we can catch easier hidden ability pokemon with at least 1-2 perfect iv stats.
Guild house daily quests with guild points as rewards that can be redeemed for pokeballs, supplies, tms even pokemon exp/ev.
Guild raid: Since it would take combined effort to beat and would be tough it will give rare epic poke as reward
Guild vs Guild pvp: Ladder with rewards at the end of the month for the higher ranked guilds
All these rewards can be additive for leveling up the guild house, for example: Guild house lvl 1 gives daily quests, Guild house lvl 2 gives private zone etc. (with scaling cost ofc like 5m for lvl 1, 10m for lvl 2). For higher lvl Guild houses the rewards can be doubled ( Guild house lvl 5 gives 2 private zones with even better pokemon- would cost like 30m).