I have a mini fridge in my garage and after adding the mini fridge I noticed at most a $2.00 per month increase in my electric bill which is not all that expensive.
Most mini fridges use between 85 to 100 watts of power when they are running and they are not running constantly because they run for about 10 to 20 minutes at a time and shut off and only come back on when the temperatures drops in the mini fridge.
Then it comes back on again to cool off and repeats the process over and over again.
So a mini fridge is a very little use of electricity and you shouldn't notice much of a raise in your electric bill each month and having a mini fridge is certainly worth it.