With a 300 watt amp, a ten inch driver plus the cost of the cabinet build, I really doubt it's possible. IMO, anything within the 500 and under range that gives excellent performance compared to the competition (SVS or BK here in the UK) is really hard to make for less if at all possible.
You may be best placed to scout the classifieds for a pre-owned sub to get the most out of that budget. I stand to be corrected though.
Thanks very much for the reply, I have the pb10 which he liked very much but he's on a budget. I have the tools for cabinet making so no problem there. Is there any link for the construction of CSS SDX10 and 300W bash amp for the enclosure dimensions?
Edit: Sorry for asking for the link, I found it. Thanks