Usually, the price you pay for the  property is what the lowest per sq ft listings appear to go for on MLS.

Purchase price: $101,000 Cash invested: $25,268 Sale price: $202,000 This was my first ever solo project.

Even with that, I think we will still be way ahead. 2 bed, 1 bath, 1200sq/ft, detached 1 car garagePurchase Price: $38,000Repairs: $20,000ARV: $96,000 (based off Zillow, but a local real estate agent thought that was pretty accurate)Mortgage: $430Insurance: $55Trash Included: $25Rent: $1,000We are now going through applications for our first tenant. 

Purchase price: $1,200,000 This is a 100+ pad community located near Downtown Bloomington (Near Indiana University).

Homes with that price range will attract bad renters.

Purchase price: $75,000 Cash invested: $10,000 Sale price: $120,000 This is a 2 bedroom 1 bathroom single family detached house is South Georgia.

When you go to sell, you're subject to tax on the amount of profit between your adjusted basis and sale price, not your purchase vs sale price.Recapture is not repaying the depreciation.

Is $3500 a fair price for the treatment?

I think a good starting point would be looking at the comps in your area (recently renovated or new construction homes) that sold around your desired sale price and in your desired timeframe and taking inspiration from there design choices. 

You will need to justify the sale price of those assets to the new owner at their then current value on your books.It is a big hassle to do this, but it can make you a lot of money if you do it right and are careful in your record keeping.