I used to be in the "nothing" camp, but having stayed offsite now a few times I change my vote to "cost." When factoring the costs of staying offsite, I include transportation (either rental car or rideshare costs including tips) and parking fees (if we're not using rideshare or if we'll have to pay them at the resort) and I try to compare resorts with similar amenities (at least the ones we will use).
$1500 is a lot of money and I wouldn't think twice about saving that and booking offsite but before you do:
-be sure that you've added up your transportation costs (as you won't be taking Disney transportation to/ from the airport if that's a factor, you won't have free parking at the parks, and you won't be taking Disney transportation to/ from the parks). If pricing rideshares, be sure to include the additional $$ for a car seat and be realistic about the size vehicle you need, and factor in everywhere you would need to go.
-be sure you're comfortable not having on-site guest "perks" for booking ADR and FP. I was surprised at how little I cared and how successful I was getting FP I wanted at 30 days out but not everyone likes to plan that way.