I think you're missing the point. I asked whether the figure was about right, not for a lecture on ethics. However, since you're so concerned, let me reassure you that because there are 2 adults involved in this scenario, we'll reach a mutually agreeable result.
I'm not concerned sunshine.
Yes the two 'adults' involved in the scenario are;
A - You, the cheapskate landlord, who is so naive as to check wether a price is fair after a job is done, then considers welching on an agreed price.
B - The installer, who will hopefully bury his largest wrench in you oversized head.
Just confirms my policy of NEVER working for landlords.