Cost doesn't set the amount of points you'll need to spent on skill but a threshold under which u cant learn it (still cost 1 point).
Example: cost set to 2, if u have 1 point than u can't learn it, if u have 2 points than u can learn it and will left with 1 point (instead of 0).
Here is definition that I used.

"luck+0.25": {
	"cost": 2,
	"title": "+0.25 Luck",
	"icon": {
		"type": "effect",
		"data": {
			"effect": "luck"
		}
	},
	"rewards": [
		{
			"type": "puffish_skills:attribute",
			"data": {
				"attribute": "generic.luck",
				"value": 0.25,
				"operation": "addition"
			}
		}
	]
}

Fixed in 0.4.0

Thanks for reporting.