The price of gas is cheaper/lower

either one is acceptable.

Tanguatlay, you are correct. A price is a sum of money which can be lower than, equal to, or higher than another. The commodity or product itself can be more or less cheap.

Petrol costs $3.00 per litre in town A, and it costs $2.75 per litre in town B. The price of petrol is lower in town B than it is in town A. Petrol is cheaper in town B than it is in town A.

"Gas" in this context appears to be what an abbreviation of "gasoline", the American word for the BrE "petrol".

Thanks, Contrex for the detailed and crystal-clear reply.

Thanks also go to Farmerman.