mello roos usually isn’t value based, so there is a set fee per year per house. Depending what county the house is in, you may be able to look at the itemized tax bill online, then run neighboring properties to confirm that line items are the same regardless of purchase price.

In riverside county you can look at anyone’s tax bill online, see if it is current and see each line item and it lits a phone number for each so you may inquire directly. Mello roos is a broad term, some of the fixed taxes may not be exactly mello roos, some are voter passed bonds, some are community service districts that can actually go up a little each year. I just looked at the fixed taxes for my new purchase and figured out when each would drop off and which ones never drop off and which ones have the possibility to rise or fall. Developments where the builder goes under have the possibility of having bond payments rise since the total debt is divided amongst lots, if the builder stops subsidizing the vacant lots and new homes aren;t built, the existing owners have to pay more to satisfy the bond. More specifics and I can probably help, but the answer is you are right, his taxes will not be halved, he will pay about 5500 a year for a 250k property, thus making his effective tax rate rise to over 2%. just multiply the diference from the previous sale to his by 1.05 and subtract from the previous bill, that will be close enough.