If you have concerns of professional trespassers then wifi is a risk as wifi jammers are readily available these days.
I would use direct burial ethernet cable but you would need PoE cameras that get power through the cable and possibly repeater switches is a length is more than 100 meters.
Negotiating a price is common practice on certain products.
Requesting a discount because you are paying with cash is a different but related situation and its not something I have seen as a common practice.