You need TWO to upgrade a lv 5 into a lv 6.
So you need a base + 2, i.e. 3.

No, I think it's probably a worse return on investment than lucky board(which is ~175% RoI compared to discounted stuff and ~250% compared to base price).
Unless, of course, you get get top 3 place cheaply. Which you can find out on the last day of the event if you could. I find it unlikely thou.

For most players, gaining 2 lv 5 magatama would be equivalent to upgrading 3 lv 4s to 3 lv 5s (remove 2 4s and use them to upgrade another 4 to 5)

This is a gain of:
60 life, 39 atk/nin, 27 def/res
Which is roughly 2/3 of what you would get if you upgrade one of each magatama from lv 5 to lv 6
OR roughly equal to upgrade one of each magatama from lv 4 to lv 5
depending on where you are at on the magatama train.

A set of lv 4 would cost, at regular price: 621 coupon. Thou most people would get them through various events etc, so probably real cost value of around 500.

Upgrading from 5 to 6 would require 6 sets of 4s, 2/3 of that is 4 sets, so a value of 2000 coupon at expected discount and ~2500 at base price
Upgrading from 4 to 5 would require 2 sets of 4s, so a value of 1000 coupon at expected discount and ~1250 at base price.

Therefore, I will say that the value of 2 lv 5 rainbows are between 1000 to 2500 coupons.

This is, of course, for the "average" player. ptwness will greatly influence this calculation, as the upgrade a ptw player is expected would differ.