Here's my findings and opinion...
1) Other variations are taken: PlanoApartment, ApartmentsPlano, ApartmentPlano, WestPlanoApartments, ThePlanoApartments, MyPlanoApartments, MoradaPlanoApartments, and LatitudePlanoApartments..
2) Similar sales: CharlotteApartments.com (Population 879,709) for $12,322.. MemphisApartments.com (Population 628,127) for $12,101.. RaleighApartments.com (Population 469,124) for $7,109.. HartfordApartments.com (Population 120,576) for $5,000.. RenoApartments.com (Population 268,851) for $4,510.. WilliamsburgApartments.com (Population 15,590) for $2,200
3) Similar domains currently for sale: ApartmentsPlano.com for $2195 & PlanoApartment.com for $375 & LatitudePlanoApartments.com $1299
If you're willing to do outbound marketing and can find the right buyer, this name is probably worth somewhere between $2500 and $5000 when you look at previous sales for domain names in similar sized cities. However, if anyone were shopping for names and doing searches on a site like GoDaddy, they would easily see the non-plural version (PlanoApartment.com) is listed on Afternic (which also shows up in GoDaddy searches, DynaDot searches..etc) for just $375 - much cheaper than the price I believe it's actually worth. You can also see that a similar variation (ApartmentsPlano.com) is listed for $2195 - cheaper than the price I gave you. With all of that in mind, I would probably undercut the later variation and list this domain at around $2000. There's no way you're gonna be able to undercut the $375 and still make much profit. If you do outbound marketing maybe quote people $2000, but if you get a smaller offer (of say, $1000) you could accept it and still be making more profit than the person who owns the non-plural version. Otherwise, consider making him an offer and owning both versions - then you can sell them together as a package and maybe get $2000 for each..
Hope it helps!