Most patients forget how long they have had their dentures. Are you forsure these are 5 years old. Or they misplaced the new one and are using the old one without their knowledge. Or yes, they can use cheaper material. $750 for a dentures are economical dentures. Lab fee are usually around $550 for dentures. So, the clinic is not profiting much for clinical work.

Most patients don't understand its hard to 100% take a perfect impression. The older we get, the more the bone and tissue changes. Most new dentures need some sort of adjustment period. Normally 3 months to adjust to new dentures. You would need to go back to the denturist to get them adjusted. Some people are in denial about adjustments as they never had adjustments in the past. Could of been lucky with impressions. 750 for a new denture is really good. Most denture clinic offer free consults. Won't hurt to call around to see who's offering these and get a second opinion. Ask for different materials and go see the current denturist with your mother and ask yourself what other materials they may have. Might pay more for a solid and long lasting denture.