Why not the advanced vendor jewels? +7 is safe. +8 is not, with vendor gems, so stop as soon as you succeed. That leaves just levels for IS jewels.
This is solid advice. However, the time and frustration rate of vendor jewels will keep many players from doing it.
I think this is the cheapest overall strategy, but it usually takes me an hour or two (of hating life) to pull it off:
vendor jewels to +3
diamond jewels to +5
perfect to +6
vendor jewels to +8
diamond jewels to +11
perfect jewels (if they existed) to +12
I just plus'd up an item to +6 yesterday... abysmal stats: It took 605 vendor jewels and 33 diamond jewels to get to +5. Those stats are what make this process so unnecessarily frustrating for players. I got to +3 at least 6 times, but then I dropped back down with diamond jewels.
Hell, raise the damned prices if money is your issue, but take the frustration out! (which could be interpreted as only sell perfects)
I wrote an addon to track plus rates and run some statistics... the vendor +6 jewels have a 20% success rate... which means less than 1% chance of getting a run of 3 in a row to get to +3. Most players do not have the patience for that. Unfortunately, the diamond +6 jewels are only 40-45% success (less samples). The vendor advanced jewels are even worse: 1-2% success rate. Who would sell an item with a 2% success rate? Really? it makes it impossible to use above +8, so why not just say that on the stupid item, raise the price, and make it a 100%??
Also, I have found zero evidence that jewels have "moods" and that there is any correlation between uses of them, nor that the current plus level has any affect on the probability of the jewel. The probability density function is uniform as far as I can tell, and each event is independent. This is the same sort of superstition that keeps casinos in business. I dont think the programmers are smart enough to make it any more complicated than if(rand()%100 < INCREDIBLY_LOW_RATE) { success() }