Current prices are available at LithiaToyotaParts.com
I usually sell these with the harnesses to make the conversion easier, so the part numbers are a little different than what you've linked to. The price is about $17 higher per lamp with the harnesses. Harnesses and bulbs are the difference between the "assembly" part numbers below and the "lens" numbers in your link.
81550-17110 - Lamp assy, RH
81560-17150 - Lamp assy, LH
81551-17100 - Lens & body, RH
81561-17140 - Lens & body, LH
81496-20010 - Guide (4 req'd = $10.92)
81670-17020 - Backup lamp assy
90179-05004 - Nut (4 req'd) (was 90179-05095)
90168-50050 - Screw (2 req'd)
You can choose US or JDM corner lenses (obviously you do not need both):
81750-17040 - Side lens, RH, US version (red)
81760-17040 - Side lens, LH, US version (red)
81750-17050 - Side lens, RH, JDM version (amber)
81760-17050 - Side lens, LH, JDM version (amber)
Here are the center panels:
75082-17050-A0 - Panel, Super White II (040)
75082-17050-B0 - Panel, Steel Mist Gray (187)
75082-17050-C0 - Panel, Black (202)
75082-17050-D0 - Panel, Super Red II (3E5)
75082-17050-F0 - Panel, Super Bright Yellow (576)
75082-17050-G0 - Panel, Dark Green Mica (6M1)
You can have a lock cylinder for cars with or without Toyota security system:
69055-17130 - Lock cylinder w/o factory alarm
69055-17101 - Lock cylinder w/ factory alarm
There are two MR2 emblem choices for the tail panel:
75471-17110-B0 - MR2 Emblem, gray (for everything but SMG)
75471-17110-B1 - MR2 Emblem, dark gray (for SMG cars)
These brackets and hardware come with the new center panels, and are also sold separately (useful if you buy a used set that doesn't include them):
76826-17010 - Retainer (bracket), RH
76827-17010 - Retainer (bracket), LH
90159-50299 - Screw (2 req'd)
90109-05034 - Bolt (4 req'd)
76817-17010 - Clip (2 req'd)
You can reuse the nuts that hold the existing tail panel to the body, or get them new:
90179-05004 - Nut (4 req'd)
Long story short, a
complete conversion reach $600 or more, depending on which taillights, which corner lenses, which center panel, which lock cylinder, and whether you want an MR2 emblem or not.
There are often cheaper "kits" for sale, but be aware that it is up the the buyer and seller to agree on what is required. Toyota does not package a kit. That means that some people are piecing together bare-bones setups and selling them as "conversion kits" with attractive prices, but you may not be getting wiring, brackets, hardware, lock cylinder, or who knows what else. Shop carefully. Know what you are buying and what else you will need before you decide that someone else is "cheaper".