i plan to invest $100 weekly for 5 years ($26,000 total) to average market entry.
Op I think you are getting yourself confused,
Weekly investment you said : $100
Total for a month investment: $100× 4weeks = $400
And I think you yearly should be : $400 × 12 months = $4,800
In your previous calculations it seems you had an error, just like you said investing $100 per week would indeed result to a yearly investment of $4,800 and not 26,000 so take corrections of that.