Always better getting and Air-con Specialist to do this.
The omega takes 950g (+- 50g)of R134a, any air con system should always be drained, vacumed for a very minimum of 30 mins, refilled with the correct oil, noting the air con compressors on Omegas use a far thicker oil than than the normal stuff, put in the dye and then the correct weight R134a.
Then once the above is done normally the operating pressure of the system would be checked, the level of cooling and the correct operation of engine fans.
If all the above is done every 2 years you will have many years of trouble free air-con.
You can't check how much gas is already in the system using a can, and that includes the one with a pressure gauge, as the pressure is always changing.
The cans are good to maybe bring life back into a system in an emergency but they don't in any way match what any air-con system need every few years, a good service that's even generally missed by dealers.
Note most dealers will get a Air-con specialist in to do any work on car air-con system anyway.
It doesn't really come down to cost a can of 400g gas from halfords will cost 40 pounds, air-con specialist will be around 40-50 pounds, it comes down to convenience and how long you intend to own the car.
Generally the omega air-con system will be trouble free if serviced every 2 years. It's better to service and keep it running than to repair a broken system as it will normally be costly and why people normally don't bother.