must buy a product when buying something

[nmeijs]

Hi,

Following thing i try to achieve, is this possible ?

I only sell 1 product

Buy 1 = $100 total
Buy 2 = $110 total
Buy 3 = $120 total
Buy 4 = $130 total
Buy 5 = $140 total
Buy 6 = $145 total
Buy 7 = $150 total
Buy 8 = $155 total etc etc

All products are monthly subscriptions

So if you only buy 1 you pay $100 

2, 3, 4, 5 are $10 each

6, 7, etc. etc. are $5 each 

The reason for this is that there must be an installation/configuration fee in the product so that included in the first product. Or can i just do an mandatory product connected to the first product ?