RE: SP Skills - it is based on the cost of the unit that comes with the skill. So by ‘240’ it means what the original unit with the skill would cost to get the level 3 version of. Note when it is inherited to ANY other unit, there’s 50% increase for the new unit to learn (so it would be at 360 for a 240 skill). Yellow border skills are 300 for the original unit, and 450 for unit that got with inheritance.
Assists - you will need to adjust strategies to use rally skills, as those are more expensive than movement ones. Particularly the duel rally plus skills are best, and non-plus duel rally being the next best option.
Best F2P arena units include mostly Gen 3/4 units like Ross, Valbar, Echnida (spelling mistake) - like armors and infantry. Grails units include Astram, Itsuki, Flame Emperor, and really any modern GHB/TT unit. Most fliers need a duel skill, along with any non-armor range unit. All cavs are not worth it, unless they are a legendary hero with duel skill built in.
ALL Healers are also not worth it either. Even with a duel skill and a dual balm special, they still fall short since the vast majority do not have a preference weapon and have less access to skills - and often their fodder is super expensive and hard to acquire with orbs. Even the most optimized healers score a whole 6 points less than any other infantry unit with a duel skill and an optimized set.
These skills need to be on all units, including bonus unit to get the highest range of scores. (probably best to have different skill sets, one for arena, one for non-arena, etc).