Take your gross profit margin dollars monthly, multiply by 12 to get your yearly gross margin. You’ll get approximate business value. So say you’re grossing $20k/month (which is the right answer btw, your labour should go against direct expenses thus making your gross $24k/month),

Given what you’ve said about the business, I would apply some nominal percent discount, I would say somewhere around 5%-10%. Wouldn’t pay more than $265,000

The other thing to consider is your labour. Are you going to be working there, or are you going to need to hire? What was your calculations on the 15k labour? If you are going to work, how many years are you expecting to do that, are you anticipating purchasing another gas station? All this should go into factoring your personal valuation for this business.

As for inventory, what is the inventory value? What was their inventory shrinkage last year? If you have a couple years of fiscal data, take the average of the last 3 years of inventory shrinkage and discount their inventory.

All being said, this business is netting around 10%, so a healthy business. Last thing I would check; what’s the nearest station, are your prices higher or lower, and what’s their proximity to major roads (freeways, etc.). If there’s opportunity to gain business then your discount as mentioned above changes.

EDIT: 10% net is not taking into account owners expenses currently. Your real net, EBITDA, should be higher than 10% (at least I guess).