You can take 10 on the check. Since the base DC is usually 10 + ranks required, you may only need to roll when you try one of the special modifiers as above.
1 skill focus is +3, a '+2/+2' feat is +2, something like educated for magecraft/psicraft could be balanced giving +1 to all such skills. Add in synergy for highly related craft -or- knowledge skills, that raises it to +8. With a say, +2 int bonus, that's +10. Using the +15 modifiers will be quite tough.
Allowing for both Artificer and Artisan to be rolled into 'take 10' without issue by a non-Wizard.
The Rapid and Efficient are supposed to be rough, since they mimic Epic level abilities.
The formula thing for constructs is ultrarough. A feat granting a +4 bonus to such int checks, with an epic version at +8, is probably not out of the question (DC 50 and a decade to make the first of any colossus!)
Permenant/unlimited effects need to be re-thought out, as well as uses per day. Need to take duration into account.
Anyway, for a comparison, armors, ability boosters, some jewelry and most wondrous items cost the same, XP wise. Weapons and some jewelry cost half. I've eliminated the absolute need for gp because, frankly, it's a stupid concept unless you decide to develop something like orichalcum as in Shadowrun, and I figure DMs who wish to do that would rather just, well, do so.