hmm... it was the same case with me... because my parents wanted to give me the mbp not mb... but you know what? i don't play that many games, not the really complex kill everyone 3d games anyway. i don't really do video editing and though i do like photoshopping, its not the end of the world if i can't have that magical graphics card which seems to me (in my very non computer proficient way) the main diff between mb and mbp- oh and the screen but i always thought 15' was a bit big (now everyone are going to tell me mbp and mb are the same size since i haven't actually seen them side by side)
i told my parents for my needs i only need a mb and basically said i wanted the higher end white one because they gave me a black i pod and i ended up getting a white case for it (doh!) and i listed out all the differences i can think of between the two and the fact i want to have the possibility of bring the thing to the library and that they are saving big bucks on a mb for my needs and they said yes. and stuck 2 gig of ram in for me and threw in a ms office. and feel that they got a good deal all round as i also gave them price comparisons with comprable ibms, fujitsu and sonys at cost (the others we were considering)
i don't know. if you really want a mb and tell them exactly why the mb is better for you then i don't think any parent will mind not shelling out lots more? and may be you can get them to get you something for the money they 'saved' like an ipod, a psp, a new sound system, new clothes whatever...