The Connector is never included under any 100cloud plans, bundles or On-Prem, so it is always a separate purchase. You are correct on the pricing for On-Prem, but if the customer is on a Gold or Platinum plan, then the Designer module is free.
The only other option is to opt for the SI Subscription Bundle, which is based on number of companies:
$99/mo for 1 - 2 companies ($1,188 annual investment)
$129/mo for 3 - 9 companies ($1,548 annual investment)
$199/mo for 10+ companies ($2,388 annual investment)
These bundles include unlimited Report Managers, unlimited Report Viewers, Designer module and Connector module.
You need to do the math to determine if it is a better deal. (Note: you don't have to be on 100cloud to choose this with On-Prem.)
On 100cloud adding 1 more Report Manager is $600/yr, 2 Viewers would be $240/yr, and Connector would be $360/yr., so total is $1,200/yr. At the 1 -2 company bundle you would save.
If the amounts are close, the subscription bundle also may make sense, so you don't have to think about how many licenses you have, thus saving time.
This is all dependent on whether SI is going to be the standard report writer long term, which I have heard rumors it may not be . . .
And, of course, Sage may figure out that the SI subscription bundle is too good a deal and discontinue that.
By the way, for the reseller, SI bundles are great for tier additions: $1,782, $2,322 and $3,582 for the three levels based on 2-year calc. Not sure if they will calc for 2019 based on 3 years.