@James Hill thank your for sharing the link to the estimating tool.... very helpful.
@Lian Shi responding to your initial post, you answered your own question. You need to know costs in order to make an offer. If you aren't willing to learn about rehabbing costs, you wont be able to determine your costs to complete, and resulting max purchase price.
To ask for a "price per sq ft" to estimate a rehab is being lazy, and dangerously so. Spend the time and learn costs. How? Be resourceful, make phone calls, talk to people, look up material costs, read on how projects are completed and what materials are required to complete it. How much does a particular contractor get paid per hour? How many hours does a particular job take?
Throwing my experience in on average rehab costs- especially in Maryland.
A typical 1950s built 1100 SQFT Rambler with basement was costing me about $60K when first started in 2010, now it's costing me about $75K. $70K contractor actual quote plus $5K in punch list, home inspection list, and other misc rehab related costs.
This is for a house that's been vacant 1-3 years, completely uninhabitable, needs basically everything and medium-high end finishes.