You could always try and set up your own personal proxy servers.
Company's like Burst.net will set up 2 IP addressed VPS's for about $6 a month and you can always get addon proxies for those. Use software like squidproxy or tinyproxy on a linux vps.
FWIW I personally hate burstnet, but if you're being price sensitive and don't mind doing a little configuration of your own, thats possibly a cheap way to do it.
I'd be more inclined to use Amazon's spot instances on EC2 and create a custom AMI which is just a proxy server. Then write a script to spin up instances (probably micro ones) when you need them, assign elastic IP's to them and away you go.
That way you're paying between 0.7-1cents per hour per proxy and can run 100's of them at a time when you need, all within a few minutes. (EDIT - the downside of this though is that you also have to pay for traffic, but its awfully convenient)