Multiplayer:
This is the plant whose value varies the most widely from beginning to end and from game to game. It takes a while to become efficient, and may not ever be if 24 is also in the game. Being a 6-cap that reliably appears before Step 3 is worth a lot in itself, though, and this plant becomes godly when 24 is never in the game. 25 & 26 are the most valuable plants on average, but nothing beats 30 for peak value. I’ve been in a game where 30 was legitimately worth $100+, which is something I’ve never come close to saying for those two plants.
Let’s just get this out of the way first: On “normal” maps, 30 is untouchable in the earliest turns. Trash is much too expensive and you won’t be close to the 6 cities needed to get proper value from the plant. 3-player is especially rough for it due to trash’s lower restock rates, and I don’t like it there until the game is close to Step 3. It’s still worth getting for cheap earlier if you have other good plants and can just sit on it, but that’s not so common. Things get a lot better in 4-player, and I would start looking at 30 around turn 5 or maybe 4 if the 24 isn’t around. Even if 24 is present I would still get 30 for cheap in Step 2 if I had no endgame plants or had great plants and could afford not to use it right away. It’s not worth a lot more than list price in these spots, mind you. Things change in 5-6 player due to trash’s higher restock rate and the greater importance of early 6-caps, so I would go for 30 on turn 3 and maybe even turn 2 as long as it doesn’t cripple my building.
I would begin paying more for 30 in 3-4 player once it costs $12-13 to run and is the only trash plant around. It can be worth $45-60 in the midgame depending on trash’s price, the price of other resources, the power plant market, and your need for a 6-cap. As a rule, 6-caps are most valuable to players who have no endgame plants or only a 4-cap. The former is because you won’t get 3 turns of Step 3 to catch up in capacity, and a 6-cap gives you a lot of flexibility when choosing your final two plants. The latter is because you can make money from Step 2 expansion and will only need a 7-cap in Step 3. These are the sorts of situations where I would be willing to pay $60 for a decently efficient 30. I would also recommend taking a look at the Plant 25 article for a discussion on combining an early 5-cap with an early 6-cap. In 5-6 player, on the other hand, this is one of the most important plants due to the emphasis on early capacity. Expect it to go for $60-70 in the midgame, though it deserves a bit of a discount in 5-player if 24 has been running.
The above mentioned situation where you should pay $100 for 30 is when trash hasn’t been used the whole game and both coal *and* oil are at risk of buyouts. I’ve been in one of these games and was kicking myself for giving up on 30 in the $80s during the mid-late game!
If you get this plant in Step 1, start Step 2 right away if you can! In fact, I would pay less for this plant if I can’t build to 7. You’ll be screwed in turn order with this plant regardless, and you need to bump trash’s restock rate.
Resource management isn’t too bad with this plant unless 24 has been running throughout the game and 38 shows up in Step 3. Trash can be in real danger of a buyout at that point, so you may have to start treating those plants like 20/31/36.
2-Player:
Similar issues to 3-player since trash restock rate is the same here, with an increased emphasis on efficiency. This plant becomes a nice buy once you get close to Step 3 and trash reaches a decent price, or if you have a bunch of money and can just not run 30 for a little while. Aside from that, nothing to be thrilled about since you’ll get so many better 6-caps in Step 3. It’s mainly a factor if you think the game will end earlier than normal. If you’re vying for this plant and you know Step 3 is coming on the next turn, just compare it to the other 6-caps on the market in terms of plant price + resource costs. It is also one of the best 6-caps available in Step 3 if trash hasn’t been used, due to its low starting cost & turn order advantage.
man in motion going nowhere 
I don’t think #30 ever works on turn 2. The trick with an early #30 is to buy it precisely on turn three, as long as the #24 is gone, and you’ve built to and will be powering at least three cities (preferably four). Jjust let it sit idly with nothing in it until it becomes profitable (usually that’s about two turns later). Sensible players won’t buy the #6, #14, or #19 as anything other than a market mover or pure desperation, since it hurts them even worse than it hurts you, so your prices should come down. If you have built in Wein on Central Europe, this can yield a dominating advantage (you just got an endgame plant that probably won’t run out of fuel at face value). On other maps, or without Wein, it’s still a nice-to-have.