This is a table of every manifestation of LP costs (only a select few songs, primarily the beginning songs, use any values outside of 5/10/15/25), their relative exp nets, and the ratios of the two (in parentheses). Generally speaking, the higher the ratio, the more LP-efficient the song is. Also generally speaking, EX charts are by far the most efficient, netting 3.32 EXP per LP spent, but they are also obviously the hardest to clear. Most Hard charts, at 15 LP cost, are also rather efficient, at a rather formidable 3.0666... EXP per LP.
The primary use of this chart is for rank leveling efficiency purposes, i.e., choosing to play a song that will get you as close to your next rank without overflowing, and then clearing an EX chart, which can put you as far as 82 exp ahead in your next rank with full LP.
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LP EXP EXP/LP
4, 9 (2.25)
5, 12 (2.4)
6, 14 (2.̅3)
7, 17 (2.̅4̅2̅8̅5̅7̅1)
8, 20 (2.5)
9, 25 (2.7)
10, 26 (2.6)
12, 35 (2.91̅6)
14, 42 (3)
15, 46 (3.0̅6)
25, 83 (3.32)
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So far, every event-exclusive song (e.g., ones that cost tokens rather than LP) seem to grant EXP as if their LP costs for each respective difficulty were 5/10/15/25. They may in fact carry such LP costs in the game data, but since they are paid for with tokens, it is impossible to tell unless they later intend to introduce them as regular tracks.