Several different issues. What do you mean when you say "mobile sound setup"? How mobile is it and what is included? I'm thinking in terms of
power source,
power distribution, transportation,
etc.
You budget is really pushing it. A recent thread over on PSW involved a discussion about trying to create a
system from scratch for 150 people in a bar for under $5,000. It can be done by lowering expectations, but a recommended
system for sound providers in that market is more like $15,000 to $20,000. Go from 150 people indoors to 200-400 people outdoors and those costs can go up quite a
bit.
That leads to what you have not mentioned. Things like mics, mic stands, cables, cases, monitors and
monitor amps,
system processing,
power distribution and so on. Probably also a box of miscellaneous adapters and connectors, tools,
gaff tape, cable protectors, maybe a sound
level meter,
etc. For outdoors add tarps, a tent or pop-up for
FOH and so forth. Lots of little things that start to add up in addition to the big ticket items. And don't forget to consider insurance, both on the equipment and for your liability.
The cost is also very dependent upon your expectations. What type of music and what
level of quality are you envisioning? If you are going to be designing your own speakers you better have some pretty specific goals in mind as far as patterns/coverage, output, sensitivity, response,
etc. in order to have a basis for the designs. And I agree with the others, unless there is some specific need that nothing out there meets or unless you account for the value of the experience and do not account for your time and effort, there is no way you can
build your own speakers as cost effectively as buying them. And if you are planning on renting or charging for the
system, people typically want and will pay for known names and using an unknown, home built
speaker could be much less acceptable.
It's not that it can't be done but rather what can be done within your budget and how that matches your goals.