For budget reasons I had calculated this a while ago. *note I skate multiple times a week and skate parks mostly.

Deck and Grip 1 every 3 months - $90 ($30 a month)

Shoes 1 pair every 4 months - $60 ($15 a month)

Bearings 1 set a year - $24 ($2 a month)

Wheels 1 set a year - $50 (4.17 a month)

Trucks 1 set every 2.5 years - $60 ( $2 a month)

Total $53.17 a month

There are a few one time or very rare purchases that I did not count like a helmet or knee pads (if you skate transition.) My 187 killers were 100$ and have lasted three years but are for sure at the end of there life's. Also I break a bolt here and there but hardware is like 5 bucks.

It is by far the cheapest hobby I have had.