Your best bet for that answer would be from a builder. It would depend all on what your choices are for those particular items. You can pick top of the line everything and it could be extremely expensive. You could be modest and pick toilets, tubs and etc. in a conservative price range.. but you also have to be mindful not to overbuild for the location.
I would recommend you find out exactly what you want and get it quoted to you before you buy. My thought here is if you buy their home and find out the addition wouldn't be cost effective for you then you shouldn't buy there home but buy something else that already has what you want. The fact that their home hadn't sold in 3 years tells me they are asking too much for it. Even though are market has been slow homes that are worth the price or are a "GOOD Deal" ARE SELLING!
If you pay too much for this home and also add on then you are just putting yourself in a bad economical position. Plus when you finance and the appraiser evaluates the home he will see they had been on the market for 3 years at "X" amount and he will automatically know IMMEDIATELY what the home is "NOT" worth. I'm just saying. :-)
Good Luck. You can probably get an average sell price in the area to see if it is a good deal before you sign papers and become committed. (plus most real estate purchase agreements have vergbiage that if the home does not appraise for the purchase price or higher then the buyer is not commited to buy the home..but by then you will have spent $375-$450 for an appraisal to find out.)
Jackie Lovell, Artisan Realtors, (317) 332-7547
Pick a number, say $250/sqft, and play with it to see if it will work:
Is $75,000 within your budget?
What will the PERMIT charges be for 300 sqft and $75,000, (ask the County Building Office)
Are you going to Run it or are you going to hire a CONTRACTOR?
What is the 300' going to be; Bed & Bath?
Foiundation; raised wood or concrete?
Shop all the materials; wood, sheetrock, flooring, roofing, framing, plumbing, fixtures, etc.; and add it up.
Finishing materials; paint, satin, mouldings. add it up.
If this doesn't work for you;
What is your budget?
Call three Contractors; you now know what to ask them, right?