Don't spend the money. Give a serious look at Joomla. For free each registered user can submit articles no problem. You can even download a free wordpress component at
joomlify that will go well with Joomla 1.0
I use Joomla for various projects and take it from me you will not grow out of the CMS and the community support is fantastic too.
Joomla!
With a little modifcation you can charge your members monthly or do it for free, but this should suit you well.