In 1991, LEGO released 3 Service Packs Technic sets, all of these sets have been retired. For that year, the total price of all retail sets cost $11, today those sets are valued around $54, a growth of 379%.
Most valuable 1991 LEGO Service Packs Technic set
5118 Flex System Elements $42.07
Most owned 1991 Service Packs Technic set
5099 Pneumatic Valves
3 sets

5099 Pneumatic Valves

Theme / Subtheme Service Packs / Technic
Year 1991
Pieces 1
Availability Retired
5099 LEGO Technic Pneumatic Valves
Retail $2.25
Value $5.76
Growth  +156.0%
Annual growth 2.5%

5118 Flex System Elements

Theme / Subtheme Service Packs / Technic
Year 1991
Pieces 35
Availability Retired
5118 LEGO Technic Flex System Elements
Retail $5.99
Value $42.07
Growth  +602.3%
Annual growth 5.7%

5272 Cylinder Motor

Theme / Subtheme Service Packs / Technic
Year 1991
Pieces 9
Availability Retired
5272 LEGO Technic Cylinder Motor
Retail $2.99
Value $5.99
Growth  +100.3%
Annual growth 2.9%