I hear what you're saying, but look at it this way. Raising from D- to D+ has a 250 spark difference. That can be done in a fairly reasonable amount of time and will in most cases be worthwhile to do.
Raising from D+ to C- is only an improvement of 100 sparks, and means with extreme costs of living is not at all worth doing unless you can do it in 1 or 2 days.
D+ needs the equivalent of three B+ common skills and one D- common skill.
From D+ to C- needs only a minimal amount of sex experience (from F- in all sex skills, reaching C- in gangbang will bring up all other needed skills to an adequate level, so effectively less than a day of training for the sex portion). Also, mature or young slaves may have these skills from the start.
C- also needs the equivalent of five B+ common skills (for example two B+, three C-, three D-). By spreading out training across more skills, you can reach the needed level faster because lower levels require less training to reach. You usually can reach the necessary levels of common skills in parallel with building up to the necessary level of obedience, so it doesn't add more training time. It might add more cost, but it's "free" if you have an assistant or can train the skills yourself, and D- or C- school lessons are relatively cheap.
The jump from C- to C+ is 150, or if you look at it for D+ to C+ for a full letter grade improvement is only 250 which means you have to make that full jump in less than a decade to make any kind of profit considering rent, food, clothing, rewards, ect.
What often happens with a more capable master is that the slave becomes devoted within 10 days after reaching D-, in which case you're held back only by the needed skills training. Effectively, the D and C ranks are clustered in value because you can achieve them in a similar amount of time (especially as your skills grow or you gain the help of an assistant).
C- to C+ needs only a little bit more common and sex skill. For example, adding three more D- common skills, raising gangbang to B+, deepthroat to C- and kiss to D- is enough. One day for common skills, one or two days for the sex skills. Can be less if you use some drugs.
I obviously don't have the experience you guys have, but the way I personally see it, it just doesn't appear to be financially viable with the way the prices are handled for C class slaves. It would obviously be a different case on medium where rent costs are so much lower and training is easier in general and you also have access to better masters with a plethora of strong skills to rely on, but on extreme where none of those are the case, the prices just don't seem viable. It feels like your only real choice is to rely on D+ until you can save up enough to make the jump to B. There doesn't appear to be much valid progression there.
Don't forget that you can sell slaves in the guild auction for more than the base values. Also, don't forget that every decade you can supplement your income with arena events.
By the time you are living in an apartment, on extreme difficulty, you would ideally have B+ guild reputation and be able to send two slaves to the arena every decade, but even if not, you can still get at least 50 sparks per decade from the arena. You can use Isabella as your first gladiatrix if you haven't acquired another "keeper" slave already. She's devoted when you acquire her, so she won't accept freedom when she wins the champion battle. Arena champions can bring in more than 100 sparks per decade.
Isabella has traits that give her bonuses as a pony girl to make winning races easier, so eventually she can be bringing in another a couple of hundred sparks per decade for you.
In the Outcasts, you also have a free barn. If you put an egglayer in there, she'll start producing sparks after she recovers from her surgery. That's a completely passive ongoing income stream with a one-time investment to acquire a slave and give her the oviparity surgery (you have the ongoing cost of feeding her, but it's less than the income she generates). And the egglayer can be used as another gladiatrix when your guild reputation is high enough to support it.
If you don't use any of that supplemental income, on extreme difficulty, you might need to rely more on auctions to maximize your income. I'm not sure, because I didn't balance extreme around the expectation of living solely on slave sale income. If you don't plan to take advantage of any supplemental income, staying in the Slums longer is a more viable strategy, because your costs will be lower. If you do take advantage of supplemental income, you have more ways to get it if you live in an apartment.
