Learning single spells outside of lists -- IMO, this should be much more costly than learning a spell of a list.
When you learn a spell list, you are starting with the basics of theory and working up into practical applications (i.e. the spells), and everything that you learn before aids in learning the higher level material.
You want something more points based and less luck based. Then you should definitely look at option 2.2 in RMC Spell Law. With it, there is no luck involved. You buy 3 ranks in a list, you know all spells on that list to 3rd level.
Now, a quick and dirty way to allow both the learning of lists (via Option 2.2) and to also allow the learning of a single spell without learning the lesser spells would be based on Option 2.2 and work as follows:
1) In order to learn a spell at given level without learning the spells that come before it, the character must spend at least 1/2 of his normal spell acquisition cost for each level below the spell to be learned. And he must also pay double the normal cost of one rank to learn the specific spell. This represents the caster learning enough of the underlying principles to be able to learn the spell in question.
For example: Joe the Mage wants to learn the 6th level spell Fire Bolt without learning the other lower level spells. Under Option 2.2, Joe has a spell acquisition cost of 2/*. This means that Joe will need to pay 1 DP (half of 2) for each of the the first five levels of the spell list. He can then pay double the full price for the 6th level spell (4 DPs). If Joe comes back later and wants to learn the 3rd level spell from that same list, then he only has pay double the normal cost of 1 rank since he already learned the principles that have applied.
Yes, this would mean that Joe pays a little extra to learn a spell list this way, but the choice becomes his, as he could just learn the entire list normally and save a few DPs.