Of course it depends on how much they charge, how often you would need the work done and how urgent the job is, as to whether that approach is economical or convenient.
Personally, it would take a couple of years for a big wrench to pay for itself and I have more than one bike.
If you do decide you need a wrench that can handle large jobs, then it is a better approach to have a low range and a high range wrench.
Of course it depends on how much they charge, how often you would need the work done and how urgent the job is, as to whether that approach is economical or convenient.
Personally, it would take a couple of years for a big wrench to pay for itself and I have more than one bike.
If you do decide you need a wrench that can handle large jobs, then it is a better approach to have a low range and a high range wrench.