We've considered your key business goals, your current content, your ability to manage your own content, the need for a truly mobile experience, browsers, search engine optimisation and now we have to be vulgar and talk money.
No doubt there's even more to consider, but if you've reached this point, keeping you for a further 5 minutes is probably asking a bit much. However, if any of the reasons above have ticked a box, then you've probably already made a mental note about what its going to cost to change things up.
Of course its impossible to price a project without knowing who your audience is, what objectives you have for the site - is it informational, is it ecommerce, is it simply a presence.
Also, don't think of a specific number. Instead think about how you actually budget for something like this ongoing. Ask how much you expect to spend on your website on a yearly basis. What returns would you want it to errrr, return? Can you put a monetary value to it - of course you can, any form of marketing has a figure. Look at what business you did last year through the web. If you're not recording this, you should be. Simply having a website launched isn't enough!
Consider the long term costs of the site too. In the first instance you'll have to fork out on design and development. After that, its the infrastructural costs, hosting, transfers, optimisation, maintenance, content/product updates etc.
A website spend isn't just a one off hit. It needs to be a perpetual figure, and reviewing it frequently should be everyone's New Years Resolution.
Hopefully this has provided some fuel for thought and more importantly, NOT put you off a website improvement :-)
Of course, it would also be churlish to not suggest we'd be happy to talk to you about all of these things!