An indoor arena will cost & 50,000 to $100,000 depending on drainage and construction. You will never make that sunk cost back from board or even when you sell the farm.
You really need to do your research locally to see what the local market is. Then you need to crunch the numbers and make a proper business plan and see what your actual costs are. Those include feed, labor, improvements, utilities, and a proportion of your land cost or mortgage. You may not even be breaking even on $300 a month board.
Where I live, granted its more expensive, but it’s going to cost close to $200 per month to feed an easy keeper.
So crunch your numbers first and see what you need to charge to actually break even. Then look at the average price in your area for your amenities. Then think about what your boarders need and value. If say half of them are retired horses or weekend trail riders they may be after basic care done dirt cheap. How many people will pull wet horses off a field to ride indoor in winter?
If you decide to raise prices you have to consider how much at once is fair. You could do it over a year or two, say $50 every six months to end up at whatever sum you need to break even and turn a small profit.
An extra $100 per horse, say 20 horses. That’s $2000 a month, $24,000 a year. That could be a help towards a new indoor. But my guess is you are currently operating at a loss if you haven’t got a really tight budget and business plan.