If you have ISDN, then you can get multiple BRI, or even a PRI (same BW as T1 - 1.544Mbps).
I have had a few installations using a Cisco 3620 or 3640, and their 8-BRI ISDN cards. Then I just added BRI as I needed them, and reconfigured the IOS to treat all of these ISDN links as an aggregate BW.
8 * BRI = 1 Mbps
Pretty affordable in my part of the world with BRI at $100 each per month. In the installations I have done, the ISP is a local call, so there's no additional usage costs. TheISP does have a charge per ISDN port that is populated. Costs are different in other parts of the world, so you'll want to check into this.
There does come a point where a plain old T1 is cheaper than aggregate ISDN. You can either keep ISDN as aPRI, or simply go to straight T1. In general, for performance, I would recommend straight T1. Maybe keep some of the ISDN for a backup.
You'll find at this point that the modular routers (like the Cisco 26xx and 36xx series) are great as you add and swap modules.