Why doesn't Nid have any mana cost?

Random sampling of 5, then (using Ultimate Bravery to randomize champs - skipping non-mana champs):

Annie:

Q costs 60/65/70/75/80 mana. W costs 70/80/90/100/110 mana. E costs 20 mana (all ranks). R costs 100 mana (all ranks).

Average mana cost Rank 1: 62. Average mana cost rank Max: 77. Base mana 334 (~5.38 casts to OOM) - 1184 (~15.37 casts to OOM). Mana Regen 6.0 - 19.6.

Overall - higher cost at low ranks, lower at higher ranks; more casts per mana bar at all ranks.

Cho'Gath:

Q costs 90 mana (all ranks). W costs 70/80/90/100/110 mana. R costs 100 mana (all ranks).

Average mana cost Rank 1: 86. Average mana cost rank Max: 100. Base mana 272 (~3.16 casts to OOM) - 952 (~9.52 casts to OOM). Mana Regen 7.2 - 14.9.

Overall - higher cost at all ranks; fewer casts per mana bar at all ranks; worse mana regen at higher ranks.

Diana:

Q costs 55 mana (all ranks). W costs 60/70/80/90/100 mana. E costs 70 mana (all ranks). R costs 50/65/80 mana.

Average mana cost Rank 1: 58. Average mana cost Rank Max: 76. Base mana 297 (~5.12 casts to OOM) - 977 (~12.85 casts to OOM). Mana Regen 6.0 - 19.6.

Overall - slightly higher cost at low ranks, lower cost at high ranks; fewer casts per mana bar at low ranks, more casts at high ranks.

Kha'Zix:

Q costs 25 mana (all ranks). W costs 55/60/65/70/70 mana. E costs 50 mana (all ranks). R costs 100 mana (all ranks).

Average mana cost Rank 1: 57. Average mana cost Rank Max: 61. Base Mana 327 (~5.73 casts to OOM) - 1007 (~16.50 casts to OOM). Mana Regen 7.6 - 16.1.

Overall - slightly higher cost at low ranks, much lower cost at high ranks; similar casts per mana bar at low ranks, scaling to significantly more casts at high ranks.

Lissandra:

Q costs 85 mana (all ranks). W costs 50 mana (all ranks). E costs 80/85/90/95/100 mana. R costs 100 mana (all ranks).

Average mana cost Rank 1: 78. Average mana cost Rank Max: 83. Base mana 304 (~3.89 casts to OOM) - 1154 (~13.90 casts to OOM). Mana Regen 5.7 - 12.5.

Overall - costs scale from significantly higher to somewhat lower; casts per mana bar scale from significantly fewer to more. (Notable: Lissandra's passive allows her occasional free casts, which offset the casts-to-OOM somewhat, and explains the reduced mana regen).

EDIT to summarize the findings:

Of these random 5, only 1 has higher averages than Nidalee across the board (Cho'Gath). The rest start higher (sometimes much higher - see Lissandra) at low ranks, and range lower (sometimes much lower - see Kha'Zix) at higher ranks. Which would indicate that her costs are fairly middle of the road.

In terms of functionality, Elise is the closest analogue to Nidalee. They are the only two champions who have toggled transformations, where only one of those forms uses mana.

But feel free to peruse this post for other random comparisons. I'll edit in a Lux and Zyra comparison to this post later.

Lux:

Q costs 50/60/70/80/90 mana. W costs 60 mana (all ranks). E costs 70/85/100/115/130 mana. R costs 100 mana (all ranks).

Average mana cost Rank 1: 70. Average mana cost rank Max: 95. Base mana 334 (~4.77 casts to OOM) - 1184 (~12.46 casts to OOM). Mana Regen 6.0 - 19.6.

Overall - higher cost at all ranks, though the difference is more pronounced at low ranks; fewer casts per mana bar at lowest ranks, more at higher ranks due to greater mana total. (Notable: I'm still of the opinion that Lux's E is somewhat over-costed, but that's purely my opinion).

Zyra:

Q costs 75/80/85/90/95 mana. E costs 70/75/80/85/90 mana. R costs 100/120/140 mana.

Average mana cost Rank 1: 81. Average mana cost rank Max: 108. Base mana 334 (~4.12 casts to OOM) - 1184 (~10.96 casts to OOM). Mana Regen 6.0 - 19.6.

Overall - higher cost at all ranks; fewer casts per mana bar at all ranks. (Notable: I expect Zyra's R cost will be changed to fit the current system (100 at all ranks), which will bring her max rank average down to 95, and her max lvl casts-to-OOM up to 12.46, like Lux's).

Good discussion. I especially like the part where you made up me saying Nid is OP.