I wouldn’t do it in my business, but it’s fair game to quote against unnamed competition (in my field, it would usually be comparing efficacy or performance rather than price but the ideas’s the same – 15% more effective than leading brand alternatives, for example
If they change their prices you’ll have to change your comparison
You can overcome by a little disclaimer in the T&C ‘Prices correct as of ddmmyy’ but as others have said, you’re asking for a price war and someone to counter on their literature quickly enough.
As above – do you need to be cheaper, maybe a price driven model isn’t what you want. Can you offer more for the same price and win that battle instead – and being a letting agent surely you can modify your terms with anyone you contract for anyway, so you can tailor price if you need to on a case by case basis?