I've been thinking of thieves' guilds and how different guild fee levels would affect the expected benefits to the members. The two models are 10% membership for typical "freelancing" PC thieves who go on their own adventures expect to pay for their own expenses and 50% "full membership" for the members who remain in their town and expect the guild boss to bail them out when they get caught.
The first kind of member would be mostly paying for "protection" to be able to practice their trade in the town (like a member of a craft guild). They would be backed by their guild in a guild war and have contacts in the underworld through the guild.
The second kind would expect more. They should get a guild-appointed attorney and suitable bribes in the court to remain free (or at least get a shorter sentence or fine).
Both would be looked after as investments by the guild boss. It would be in their advantage to keep them free and earning him more gold. For the second kind of thief 40% their earnings (those in excess to the first) would be used to pay for any other expenses, whatever those are.
With customs and tithes being at the same 10% level, that could be assumed to be just the price of doíng business for thieves too. For other guilds (mages or craftsmen), the same 10% could be applied too. This would give the members both the right to ply their trade and in the case of mages and clerics access to guild/temple libraries and laboratories.
What level of services are available to each member (of cult/church, thieves', magic or trade guild) could be estimated from the total investment using the magic number (33) times 10% of their monthly income. If the character has been paying their dues the whole of their career, they should have invested about 8% of their current XP in the institution. This can be thought as the maximum level of help they could possibly expect from the guild/temple.
So a fighter could expect a maximum of 1 "Restore Life and Limb" from their patron temple if they had been paying them the whole time. In practice the institution wouldn't want to pay back all of the funds. If adventurers are though of as perilous investments (9% return per month), the temples would likely pay out just 10% of the tithes as spells, items or services to the adventurer (pushing the "free RLL" level to 7th). This would be just 0.8% of the total XP accrued. In the case of safer trades, the guildsman could hope to get 90% percent return from his dues as services like health care, funerals, retirement and widow's pay.
With the monthly hijink income being about 15% of the average "eavesdropping" income for carousers and 1st level thieves, the freelancers should expect nothing in return for their 10% dues except being left alone. The "full members" of a thieves' guild paying 50% could then expect to see 70% of their dues to come back to them, making the guild a risky to perilous business. About 30% (50% the total dues) of that would get them the same kind of legal aid as a salaried thief (nothing for carousers), and the rest would be a combination of services (free rumours and accomplices), gear, accomodations in the hideout, access to healing and curse removal and maybe even some kind of retirement pay or funeral. So a guild thief could hope to have at maximum 22% of their total XP on their "good will" account for the guild. This would put the "free RLL" level to 3, assuming no other expenses (except for legal fees) had been paid before.
Finally, comparing to the taxation levels (urban 15% and rural peasants 50%) these membership fee/guild income levels don't seem unreasonable. A freelancer thief is like an urban craftsman or vagabond trader and the full member is like a peasant (with maybe better benefits, but also risks).