Edition.net are offering dedicated Xserve server solutions for US$250 (presumably per month, plus US$400 setup). For that you get “MacOSX Server featuring Xserve – 1.33Ghz PowerPC G4, 60gig storage, 512MB ram, WebObjects 5”. Although the RAM (and storage) seems a bit small. There are other options available which include WebStar V, FileMaker Unlimited (version 5?), and Lasso Pro 6 (not 8!). They do offer “flat-rate pricing with no traffic surcharges”.
digital.forest offers an Enhanced plan for US$30 setup and US$19/month (+US$10/month for MySQL) with perl, PHP5, Python, Mac OS X and more, 100MB of storage, and 3Gb of data transfer. Not sure what their colocating price is like though as you need to contact a salesdroid. Digital.forest have been around since 1994 and have a long time association with Mac newsletter TidBITs.