Given that 90 hp engines go from $700 (Mercury Optimax),$5,800 (Tohatsu) up to $7,000+ (Yamaha), the best engine price to get would be a used Tohatsu for roughly $2,500-$3,000.
The range of prices for outboard motors can starts from around $2000 for a lower-end brand, and approaches $7000 for the top-end more efficient models.
Depending on the brand and year of the motor expect to pay between six and eight thousand dollars. Todays newer motors are far superior in fuel economy so it pays to pass on a lower priced older used motor.