I used to rent my house out. Used to pay around £60 (plus VAT, if the RGI was VAT registered) for a certificate on the boiler and cooker, more if the boiler needed cleaning. Took around 30 minutes IIRC. That, I felt, was a fair price, though to be honest I never shopped around based on price anyway and this was years ago.
Now, as a plumber myself, I won't go out for less than £45 if it's local. Tried working for less but worked out that any less than that and I may as well get a full-time National Living Wage of £7.83 job. No van, no diesel, no insurance, no NI bill at the end of the year, no spending evenings quoting, and no need to worry about anything except remembering to take my lunch in, and money in my account every single Friday.
One customer did find someone 'cheaper' than me, he thinks, but I'm pretty sure the work the other guy did was dangerous and illegal, though I only saw a photo.
Generally, I get repeat custom and recommendations, so I must be doing something right.
As someone who has made significant investment in training, tools, and time, I expect to earn over the Living Wage of £9 an hour, counting all hours and after all expenses at the very least and I'm not even gas registered.
I have plenty of sympathy for those who refuse to cut corners and put others at risk by failing to do their jobs properly just so as to give people a cheap job. Tell your clients that if they want a cheaper Landlord's certificate, they may as well write it out themselves for all it would be worth, but you get the job done properly and it costs ---.