I take it by A1 you mean the full 125cc license?
A1 Light motorcycle up to 11 kW (and a power-to-weight ratio not more than 0.1 kW per kg) and 125 cc CBT, theory test, practical test 17
A2 Standard motorcycle up to 35 kW (and a power-to-weight ratio not more than 0.2 kW per kg), bike mustn’t be derived from vehicle more than twice its power Direct access route - theory and practical
A - Unrestricted motorcycles in size/power, with or without a sidecar, and motor tricycles with power output over 15 kW Direct access route - CBT theory and practical (you must be at least 24)
Progressive access route - held an A2 licence for a minimum of 2 years - practical test (21 or over) 24 (direct) or 21 (progressive access)
In any case the cost will be the same for whatever your first license is going to be. I paid about £700 to do a 5 day course inc. CBT for my A2 license. Just bare in mind you'll need to go back and redo MOD1&2 to upgrade to the next license.