I eventually learned the best model (for me) was to copy what Yoast did… charge a flat free for SEO Audits. People were always super happy with my audits. I still have my SEO audit templates (one for local SEO, national SEO, etc). Sometimes it would only take me 4 hours to write an audit and I would get $750, sometimes more if they wanted a more thorough audit. Maybe I undercharged?
On the other hand: the main reason of bankruptcy for small businesses is bad financial management. Simply said: to forget to make invoices, not checking the payments and not following bad payers. 1 out of 3 is going bankrupt for this reason alone. I had to learn it myself. I spend at least 10% of my time with financial stuff. I don’t love it but the bills get payed ;-)
To boost your chances to earn from affiliate marketing, various sites have various tools to make your campaign more efficient. There are a lot of tools which would be helpful out there. For example, if you use a WordPress site, then you should think of getting a plugin that is like the Affiliate Link Manager. Online money making is eased with the usage of right tools. 
StackPath – CDN with 31 additional data centers (Cloudflare has 150+ data centers, but more data centers = faster content delivery). I get around $1,000/month by referring people to StackPath in my cache plugin tutorials. StackPath recently bought MaxCDN and their affiliate acceptance rate is much lower (depends on your potential volume) but most cache plugins converted to StackPath as their recommended CDN
Transcription is one of my part time jobs. I can do it conveniently from home and many of the clients are trainers or speakers. Having money and needing transcription services doesn’t imply you have a secretary. At my full time job the closest we have to a secretary is the CEO’s assistant. She books flights, plans travel, schedules meetings, transports his children, and helps with home renovations. She doesn’t have the time to sit and transcribe work when the CEO can send it to a company or freelance agent for a few dollars.