It's quite simple, the content of what you need to say ... the hard part is minding your manners as you do so, and making the sale.
The short form is: My price is my price, I really don't care what anyone else has to say about it, and if you can get it cheaper elsewhere - do it!
Yet, this is an opportunity for you to stress your experience and qualifications; to explain what all is involved (it's never just 'running some wire!'). As I've suggested before here, look to HGTV shows like "Holmes on Homes" and "Tough as Nails" to see some master salesmanship at work.
Finally, if the situation is turning ugly, this is an opportunity to 'discover something you overlooked' and RAISE your price - to cover the agrivation factor.
"You can always count on Reno to offer some brothel tested advice"