Generally you get pluses one at a time, usually on cards you would just sell. Pluses are usually all fed to single powerful card to make it better.
The problem is that that's expensive. Feeding a single junky card to an high-level ultimate evo card can cost over 50k coins. What's the most coin-efficient way of moving pluses from a ton of crummy cards to a single powerful card?
Should I feed all the pluses to a junky card for cheap, them feed that one card to the target once it's +297?
Best Answer
On the bottom of this page, there is a formula which lets you calculate the exact fusion costs when plus-eggs are involved. In addition to the regular cost there is a 1000 coin cost per plus for all the pluses involved.
So we have two parts of the formula we should minimize, the regular cost and the plus-egg cost. The regular cost is fairly straightforward - just keep your target monster's level low. Ideally, this means holding on to all your plus-eggs on, as you mentioned, crummy cards with a low max level.
The plus-egg cost is a bit harder to manage. Since it depends on the total pluses after fusion, you're effectively paying for each plus every single time you use it in fusion. Thus, you want to feed them as little as possible on the way to +297. There is a table on that page which outlines the cheapest way to +297 as follows:
This results in a total plus-egg cost increase of 999,000 gold. Of course, you want to make sure that your final +297 has +99 distributed in each stat, or the fusion costs to fix that will make this whole process sort of pointless.
It turns out that there is a slightly cheaper (but much more annoying) way to do this. What I explained above is relatively spot-on, but it turns out there's a better way to get the +117:
If you really want to save another 10k coins so that your total is 989,000 coins, be my guest. If you ask me, it's not worth the extra trouble - I just mentioned this here for the sake of completeness.