I checked my NADA book. and the low retail price is $2,225, and the average retail price is $3,975. You wont get a high book price on a 91 unless its in pristine showroom condition.
Those prices are a national average. Some spots in the U.S. are higher, and others are lower.
Like Stan said, check craig's list for prices in your area. A good rough average to go by is $500/yr difference. In other words if a 94 is selling or sold for $4000, then a 91 which is 3 yrs older would sell for $2500.
'87 YJ, 4.2L built and bored, 8" of lift, 8.8 rear, AX15/NP 231J, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...