I love this idea using 100% LEGO items! Very Rube Goldberg! You are a mechanical genius.
Doing some math, though, it seems if you don't mind using aftermarket parts from (shameless plug) LifeLites you could come out tremendously cheaper.
LEGO method (prices in USD):
6.99 - PF battery box
14.99 - PF receiver
9.49 - PF remote (not shown)
2.00 - 9v battery box (rough guess on price)
7.49 - M motor 1
7.49 - M motor 2
6.49 - PF LED set 1
6.49 - PF LED set 2
6.49 - PF LED set 3
5.79 - PF reversing switch 1
5.79 - PF reversing switch 2
5.79 - PF reversing switch 3
2.99 - PF extension wire 1
2.99 - PF extension wire 2
0.00 - misc bricks and pieces from your collection
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91.27 - plus shipping for 6 lights and 1 to 2 different patterns
LifeLites method (prices in USD):
6.99 - PF battery box
14.99 - PF receiver
9.49 - PF remote
50.00 - eLite PF
0.00 - LED cables (eLite PF includes 8 LED cables)
0.00 - misc bricks and pieces from your collection
----------------------
81.47 - plus shipping for 8 lights and roughly 24 different patterns/variations
Some ways to make the LifeLites method cheaper:
Lose wireless control and go with a legacy 9v battery box. That makes final cost based on above pricing $52.00 + shipping. The LEGO method would be roughly 69.29 + shipping without wireless control.
Go with 4 lights. The eLite Jr. has a built in replaceable battery, 4 LEDs, and anywhere from 6-24 patterns/variations based on the kit you select. eLite Jr. is priced at just $20 + shipping. I'm going to guess and say going with 4 lights on the LEGO version is going to roughly save you the cost of one PF LED kit, and maybe that 2.00 battery box and an extension cable if you reconfigured everything.
Hope this information helps!