well first off what are you going to build it out of
all acrylic
all glass
ply
a combination
would a pond type consturction suffice or do you want a viewing window
Alot of the cost will be in the glass or acrylic
If you look around at the 1500gal tank build the guys total cost was 4,000$ and change. Almost half of which was his glass
a pond build will be cheaper
Then think about what your going to do for filtration
Sharks are going to need more than freshwater fish would
The guy who built his 1500gal made his own sump.
Just look around in the DIY and youll find plenty of good ideas, then post specific questions instead of one broad one. I think youll get more help that way. I do understand where your coming from however, just looking to get pointed in the right direction correct? plan out your build before you do it and check costs so you dont run out of funds part way through. Overbudget of course, you may need more epoxy than you originally though. And overbuild if your nervous, its always better to have more support than not enough. As i said, the guy who built his 1500gal tank has lots of pics so that should give you a good idea of how much support you would need for a 300gal tank, If any at all. Its also a good example because its one of the few threads where people build a big tank and have kept up with it and not run out of time or money. Its all finished now so you can check out the finished product.