These were largely introduced in the period when the government was heavily supporting the bus industry through the Covid crash in passengers. That essentially guaranteed a government top-up to make the operator break even on operating costs after considering all revenue and subsidy, set against all allowable operating costs. While capital investment and finance costs were not allowable costs (so no new buses), maintenance and repainting was an allowable cost. (Similar measures were taken to support the rail franchisees, except they were allowed a small profit; those arrangements still persisting, of course.) 

So I suspect most of the costs of the bus branding would have been covered by the government. 

While the core purpose of the government funding was to avoid the financial collapse of the whole bus (and rail) industries, it was to keep the supply chain businesses afloat too. So maybe Go Ahead were especially vigorous in giving work to its paint and vinyl suppliers ...

I have always thought the branding got completely carried away, spoiling what had seemed a decent core effort (eg the true X-lines).