A Mallard is an entry level trailer and as good as any in its class.
As for the price, don't even consider paying over NADA book. If a private sale, offer substantially less and no more than NADA low retail. If it is a no-trade transacation wiht a dealer, offer substantially less than NADA retail and try to close the deal for somewhat less than NADA low retail. If you have a trade-in, figure NADA wholesale for your trade against low retail for the newer one. You may be able to do beter than these suggestions, but you should not accept any worse deal. If the seller won't deal, go find another rig - there are many, many excellent used RVs available.