You'd be surprised, overhead in smaller centres can be just as high - but for different reasons (and not always real reasons, but just "well i can charge you because we are your only option" kinda thing) such as staff costs - its harder to get qualified MOAs and nurses etc in smaller centres, so their wages are higher. Same reason why a tim hortons worker in a small rural centre could make 25$/hr etc. Building costs for newer builds can be alot in some rural areas, but overall real estate costs are alot cheaper...but then you have scarcity of available options in some areas.
Take 30% as an average, and will widly vary. Be aware of comparables and what you actually get for your 30%. If it gets you a well trained MOA, 2 rooms, well stocked etc etc etc maybe its worth it - if that all allows you to see more patients and bill more. If it just gets you a barebone room and you have to do all your own admin work, then thats a raw deal.
In an urban setting, i can get a range from 15% to 35%, and 35% isnt necessarily better serviced than 20%. In rural settings, there is also the variable about fee bonuses, and set rates etc etc, so variable.