The walk up is roughly two hours in the dark with a torch, and the season changes how that two hours feels more than it changes how long it takes.
In the dry months the volcanic ground is firm and dusty. The loose upper section is still loose — it is a volcano, and the top of it is gravel — but it holds together under a boot. This is when the highest number of people on this site book the walk, and with 11,261 reviews behind $27 there is a very large body of experience to draw on.
In the wet months the same ground is slick in places, the lower forest section can be muddy, and a wet volcanic slope under a head torch asks more of your footing than a dry one. Guides go at the pace of the group, so it takes longer rather than becoming dangerous, but it is the reason the walk-up listings are the ones most affected by the calendar.
The jeep listings care much less. $20 and $45 drive a track rather than climb a path, so rain changes the view far more than it changes the journey. If you are coming in the wet stretch and you want to be on the rim at first light regardless, the jeep versions are the sensible way to do it. The same logic applies to $20 and $26, which use the same roads.
The lava field listings sit in between. An ATV or a quad on wet volcanic ground is a muddier, slower morning than the same run in August — $21 and $21 both carry 4.9 ratings, and the reviews behind them are written across the whole year.