I have recently come across another great proxy host: VPS.net. They have attractively priced VPSs which you can instantly upgrade on demand, and downgrade during school holidays for example.
This scalability is achieved by means of nodes, and you only pay for what you use. Furthermore can you have your VPS in the United States or UK. VPSs are available from as little as s at 20$, and the more nodes you buy the cheaper they get; CPanel is available for 10$ a month as usual.
More advantage of VPS.net include failover / high availability, burst needs scheduling and automatic backups. VPS.net has been recommended to me by a fellow proxy owner and customer of VPS.net I know quite well, yet he is not actively participating in proxy discussions.
VPS.net under the current name exists 6 month, but they have been in business 13 years. I have also checked them out. Please PM me for the VPS.net affiliate link and 25% discount code.