I'm a web designer too and I've seen rates in my local area from $20 per hour to as high as $95 per hour. I've also seen flat rates from $99 for a one page site to some really crazy amounts...I'm talking $10,000 for ecommerce.
It really depends on what she wants, what your are able to do, etc. Personally I charge $99 for a 3 page site and $30 for each additional page. I find the flat rate is an easier sell to the customer. Sometimes I come out good...and sometimes I lose on flat rates. Very important to find out up front what she wants so you don't have a lot of revisions.
I use templates/sitebuilders so it is not hard and there is very little actual coding.
Make sure to find out what she can spend on hosting. Prices for this range from $5 per month and up depending on how much space she needs.
If she needs a domain name, don't pay the $30-$75 I see so often. You find them for $9.95 and with an excellent company. I use them because the domain name gets registered to the person...not the domain company so you maintain full control over it.
I've put links below to a hosting site with sitebuilder software included as well as the domain name company I use.
Hope that helps.
why don't you ask your aunt what she feels confortable to pay - she is already acknowledged her will to pay you, and come to a reasonable cost which both of you are happy with....?
but if prefer to make as much money possible off your aunt that is a different story....
I do my own web designing for some people at school, when I do there myspace I charge about $30 per page, but my friend does web design and he charges 400 for a basic template and 500 or more for a custom site