What is the best way to estimate repair/rehab costs on vacant properties

[Tina Devine]

I am new to R.E. investing. I started a yellow letter marketing campaign and I received my first phone call today. I targeted boarded up properties. I can figure out the ARV, but I don't know how to estimate repair costs. Is there a rule of thumb, or a formula I can use?

 I'm pretty sure there's a ton of work that needs to be done I just don't know how to calculate that # so that I can calculate a offer.  Any advice shared will be appreciated. Thank you