Never really got into the whole recipe thing...Where do i start?

Not sure one a good time to start, but you don't outlevel crafted IOs. You can still slot level 30 IO's at level 50.

First off, two things:

1) You never outlevel IOs, so you can keep them longer.

2) A set bonus is never not worth it. It might not be a huge bonus, but it's better than anything that an SO could give you.

That being said, I usually start looking for set enhancements for my characters around level 30. I buy SOs up until level 22, then around level 27 I start using level 30 IOs where I can find them, and SOs to fill in the holes. By level 32, I usually have basic IOs in everything, and start looking for sets.

Even if you're slotting at level 50, the bonuses can make a marked improvement in your character, so don't discount them for characters at the level cap. Then can allow you to do things an SO-ed out character can't do.

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True, but at some point, that can become irrelevant. Especially with ED in mind, getting the max bonus from each IO in the power can become less useful. You can sometimes get to good enhancement values using lower-level IOs, because they provide a greater bonus.

It's never really too soon to start. I begin as early as level 7 to do something simple as frankenslot an IO set level 10 acc/dam with a rech/dam or something similar into a primary attack or two. The total bonuses are considerably higher, especially if you use doublets/triplets from IO sets. Typically the cost of doing this at lower levels is fairly low, as these recipes and associated salvage tend to sell at giveaway prices, although it's more expensive than slotting TO's (and later, DO's/SO's). In order to keep my character optimized, I like to do level upgrades every 5 levels, starting at 12, then 17... up to 47, maybe every 10 levels instead of 5 if I'm leveling fast. I've never found inf to be a problem in doing this. I accept the replacement cost as part of the cost of keeping my characters optimally slotted throughout their career so that I can enjoy playing them a notch above standard enhancement performance. After all, a character will spend most of their career playing below 50th level.

But it also depends on your play style and how often you level and want to upgrade as you level up. If you are racing through the first 22 levels, you might wait you get to 22 to start slotting generic 25's (which is where generics start to really shine) and starting looking at IO sets then. Also, a lot of the more advanced IO sets start at level 25 or 30, so it might more sense to wait to slot these if you don't want to replace lower level IO sets that only run up to level 30 or 40. To check these sets out, have a look at Paragon wiki under the Invention Origin tab.

In short, do what you're comfortable with and willing to spend a little time on, maybe more a favorite character than another. You'll have no regrets doing it either sooner or later as you can always do it anytime you want (and have the inf for).

The difference between a common level 30 damage IO and a level 40 is a whopping 3.8% increase. A set IO triple enhancement, like Touch of Death D/E/R? 1.9% per attribute. Once ED is taken into account, I personally feel those small amounts have very little practical in-game effect, especially when compared to set bonuses.

And then there's the price to take into consideration as well...

For a single piece, yes. But if you replace two level 30 damage IO's with two level 40 damage IO's, you will see a 7.6% increase in the power's damage. This increase may occasionally, for example, make the difference between it taking three shots to take out an enemy versus four shots, reducing the number of total attacks it requires to take out a mob. (Of course, with ED, if you slot three instead of two, the returns diminish very rapidly as it approaches the ED limit.)

This effect is even more pronounced with multi-aspect IO's. Assuming you (franken) slotted, for example, A/D/E, A/D/R, A/E/R, D/E/R and changed all those up from level 30 to level 40, you would see a 5.7% increase in all four attributes of A, D, E and R, for a total 22.8% across the board increases in a 4-slotted power. And it does have an effect on game play, as you hit just occasionally more often, while just occasionally taking fewer shots to have to take an enemy out, while having just slightly a faster recharge time to take the shot again, while incrementally reducing the endurance to do all of this. And that's just for one power. Particularly with a melee type AT like a scrapper, the effect this kind of upgrade from 30 to 40 across the board is quite noticeable in the attack chain, as it has an effect on both the player (attack rate and endurance) and on the enemy (hit frequency and damage).

Also, in some areas, such as defense, those small percentages have much greater impact as you approach the soft cap. That 3.8% near (or to reach) the overall defense soft cap is of much greater effect than 3.8% standalone.

Yet, you're right. After level 30, the returns diminish rapidly relative to the increases between 10-20-30. But you could also say those lower-level increases are very large 'jump' increases relative the the 'normal' 3.8% increases between 30-40-50. I agree, though, that there are times where these small percentages contribute less to the overall build relative to other bonuses, in particular set bonuses. I think finding this balance (and the inf balance!) is both an art and a science without any one single correct answer for a character as a whole.