There are several ways you could produce a nice, quality marker for your Mum within the budget you have. Probably cheaper and nicer than you think. View my website at
www.rocklight.org. And I can give you other resources and links. I am in San Antonio, Texas. You should be able to purchase a granite blank for less than $300 and quite a bit less depending on color and size. Check with the cemetery as many have varying restrictions as to size and placement. Some even restrict some font and stone colors. Moving the stone can be your biggest obstacle sometimes. I could create a stencil for you to apply to the stone and take to a local sandblaster for engraving. many options. Contact me with questions.