In 1991, LEGO released 2 Promotional sets across 2 subthemes, all of these sets have been retired. Subthemes for that year included Ferries and Value Packs. For that year, the total price of all retail sets cost $12, today those sets are valued around $1,360, a growth of 11234%.
Most valuable 1991 LEGO Promotional set
1548 Stena Line Ferry $439.35
2 sets

1476 Five Set Bonus Pack

Theme / Subtheme Promotional / Value Packs
Year 1991
Pieces 158
Availability Retired
1476 LEGO Five Set Bonus Pack
Retail $12.00
Value $920.68
Growth  +7572.3%
Annual growth 9.4%

1548 Stena Line Ferry

Theme / Subtheme Promotional / Ferries
Year 1991
Pieces 172
Availability Retired
1548 LEGO Stena Line Ferry
promotional
Value $439.35
Annual growth 4.3%