This isn't really a cheap build, and as far as I know I'm pretty much the only one who's ever built her like this. I figured that I'd share it though. The way that I play Diana is pretty tanky. By the time I get to the end of the game with all my items I'm in the 4k hp range, and have decent AP and defense. I realize this is probably a very wrong way to build her, but when I can solo the dragon at level 12, I feel kinda nice.

I only play Diana mid, and on a good game, I can stay there, with just level 1 boots until around level 15 (if the game isn't pushed). What I tend to do is go back when: I reach 5k, hit level 16, or die. Whichever happens to come first.

I've found that Diana is a SERIOUS mana spammer. Not as bad as some, but...she has some issues. For me, this was very easily fixed with only taking Clarity. I didn't even need any mana items after this. I never use potions, just Clarity. If you can make yourself last, then you should be able to survive on the mana that you have, using Clarity every time that it's off cooldown.

I take Heal because I'm paranoid. Even though I have a built in shield, there are still times where I panic and think "Hey, I might actually die." Having Heal can sometimes get you out of danger if you use it and then run away. That small boost of HP could be enough to save your life.

The reason that I chose these items is simply because I love being tanky. Note that I myself never buy potions, ever. So if you're the type of person to need them, than feel free to buy them when needed. If you happen to die before reaching the 5k gold mark, buy your boots first. If you do make it to the 5k mark, get your boots AND Warmog's.

Merc Treds: I get these for their right off the bat magic resist. This allows Diana to have more survivability against mids like Karthas and Annie.

Warmog's: Health. Diana is really good with health build items. I've managed to get around 4.8k depending on my final item. Once you have this and level 12, the dragon should be a piece of cake. Just make sure you have Heal as a summoner spell.

Rylai's: More HP! You always need HP! Plus, you get the added ability power on top of it so that her already pretty good damage becomes amplified. At this point, you can probably start soloing the dragon pretty quickly.

Rod of Ages: EVEN MORE HP! With the added AP and even a mana boost. Can you tell yet how I play her? HP is very important. You may be saying to yourself "HP doesn't mean anything without defense.", and I can tell you that's both true and untrue. Even if you don't have a ****ton of defense at this point, you still have that massive amount of HP that can and will allow for you to take more hits. With this you should be able to handle most situations, and it's ideal at this point to actually start team fighting, if you haven't already. Also noted is that with this amount of HP, you can pretty much tower dive and do a fair amount of damage before running away.

Zhonya's Hourglass: Here's some armor for you, and a good chunk of AP. Should be in the 200's for AP by now. 50 armor may not seem like a lot, but you'd actually be surprised. Remember that massive amount of HP? Yeah, now you can take even more hits since you got that added armor. It's also fairly useful for that 2 second invulnerability. Sticky situations and all.

Sunfire Cape (or situation dependent item): For me, 90% of the time, I'm using the sunfire cape. Another 45 armor, and yup, you guessed it, even more HP. This gives you another 450 HP. FOUR. HUNDRED. AND. FIFTY. It also helps that it's special ability does 40 magic damage to enemies.

The reason I say 'item dependent' is becomes sometimes...there are other more important needs. Such as banshie's veil. Though Sunfire Cape and Banshie's Veil are the only two last items I've ever needed to build, that doesn't mean that there aren't other things to be needed. It really all just depends on your enemy team. Make sure you know what you're preparing for.

In conclusion to this build, you have to keep in mind that it's costly, HP heavy, and meant mostly for survivability factors. This build is not completely tank, it's tanky-ap, I guess. Even then, it's not very high on AP. This is a HEALTH build. 4.8k health is pretty spectacular, at least in my opinion.