Open access has the advantage of making your research more accessible, which many funding bodies like. So if you have a grant already, you may be able to use that to pay the fee. It is really tricky if you start a small project without funding however!
It's more exact to say that open acces CAN be expensive: publishing an article in a Public Library of Science (PLoS) journal costs betweem 1350 and 2900 USD, and choosing the open access option offered by publishers like Elsevier even more (3000+ USD). But open access can also be free. On the one hand, the majority of the 8500+ open access journals listed in DOAJ (http://doaj.org) don't apply publication charges. On the other hand, an author can deposit in an open-access repository (central or institutional) her final, revised manuscript once it has been accepted by a journal. Most publishers, including all the major ones (Elsevier, Springer, etc.) allow it, though sometimes with a 6- or 12-month delay. This is called Green Open Access, while publishing in an open access journal is known as Gold Open Access.
Open access is simply a farcical name as its in reality "reverse restricted access", the restriction this time being on the research creator rather than the research user. This situation is much worse than the existing system as this means that there is now a mechanism for selective suppression of research output and this can be used for political gain - for example