First, we need to determine how many miles Tony drives in 4 weeks. We do this by performing 50*5=<<50*5=250>>250 miles per week.
Next, we multiply the weekly total by 4 by performing 4*250= <<4*250=1000>>1000 miles in 4 weeks.
Now, we need to find out how many times Tony needs to fill up his gas tank. His gas tank holds enough gas to last 25*10=<<25*10=250>>250 miles.
This means in 4 weeks, he needs to fill up his tank 1000/250= <<1000/250=4>>4 times.
Since Tony's gas tank holds 10 gallons, this means he needs to pay for 4*10- 40 gallons of gas in a month.
Since gas costs $2 a gallon, this means Tony spends 40*2=$<<40*2=80>>80 on gas every month,
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