I don’t think you can make a valid comparison if you haven’t experienced staying at a Disney property. I have stayed offsite and wouldn’t do it again.
Disney is worth the extra cost. There are lots of different things to do at Disney that have nothing to do with the parks and unless you stay at a resort you may never know about them. My kids for example loved the Horse Riding Experience. Going on a boat ride is also pretty cool.
The extra benefits on staying onsite far outweigh the cost difference. Being able to access fast pass booking 60 days in advance of your holiday, compared to 30 days if you stay offsite is worth a lot to me. I own at BLT and can walk to the park. I can come and go as I please and don’t need to worry about parking, getting stuck in traffic or depend upon outside buses picking me up.
When the parks get busy, I just take a 5 min walk back to my hotel, chill out for a bit and then go back when things are not so busy. You have a far more relaxed vacation staying on site.
As a side note some of the cheaper Disney resorts are great value if you don’t want to spend much. I would be surprised if you could buy a better option for half the price of a night at Pop Century.
You should give on site a try...you might be surprised.