In keeping with my interest in the Rogue type, I have been thinking about the way in which Rogues learn spells particularly the solo Rogue. In a group game it is simple enough especially if there is a Wizard in the party (as there should be). That Wizard starts with the Teacher spell which he can use to teach Rogues any spell. If the Rogue wants a spell that the Wizard does not know (or is unwilling to teach), the player could always seek out another Wizard through the Game Master and pay an exorbitant fee.
Solo adventures are, as always, a little different in that the player is also (effectively) the Game Master especially during the time between adventures when the Rogue would be trying to learn spells. Your Rogue may find some ancient scrolls in the course of an adventure or perhaps even learn a spell during an adventure, but most would be learned during his/her 'down time.' But how?
The simplest way to do this would be to assume that the Rogue could learn any spell he/she wants. But I prefer a bit more of a challenge. So, for each level of spell, make a Saving Roll on Luck (or Roguery if using 7th edition) to see if you can find a teacher. If you want a Level 1 spell, make a Level 1 Saving Roll on Luck/Roguery. For a Level 5 spell, make a Level 5 Saving Roll on Luck/Roguery. If you fail the Saving Roll, you must wait until the character completes another solo adventure before he/she tries to find a teacher for that spell again. You may try to find as many spells as you can afford, but you should only receive Adventure Points for these Saving Rolls if you actually obtain the spell being sought.
But how much would the spell cost? There are prices listed for each spell in the rulebook: 500 gp per level starting at 2nd level in 5th edition rules or 1000 gp per level in 7th edition. But these are the prices charged to Wizards by the Wizard's Guild. They will not teach Rogues, so Rogues are left to find unscrupulous Wizards to teach them spells on the sly. Of course the punishment for getting caught teaching Rogues magic is severe, so these Wizards would need the reward to outweigh the risk. So how much would the teacher charge? Two times as much? Three time? Ten times? Time for another Saving Roll, this time on Charisma or Roguery. Make your Saving Roll and determine your level of success. For each level of success deduct one from the multiplier on the table below; however the final cost must be at least twice the listed cost. Failing the Saving Roll simply means that you must use the listed multiplier. If you cannot pay the final price, you must wait until the character completes another solo adventure before trying to obtain the spell again (you'll have to start over by trying to find a teacher). No Adventure Points are awarded for this Saving Roll.
1st - 2nd Level : 3x
3rd - 4th Level : 4x
5th - 6th Level : 5x
7th - 8th Level : 6x
9th - 10th Level : 7x
11th - 12th Level : 8x
13th Level : 9x
Remember that according to the 5th edition rules, Rogues may not learn spells above 7th level; they're just too complicated. This restriction does not apply to the 7th edition rules. Also, in the 5th edition rules there is no cost listed for 1st Level spells. For this House Rule, assume that their base (Wizard) cost is 250 gp.
So there you have it, a simple House Rule involving only two Saving Rolls to determine what spells your solo Rogues can find a teacher for and how much they'll need to pay to learn them.