Keep it simple.....

If you don't have a magical money tree you will have a budget you would like to keep to. 
Always have some contingency, usually around 10%, in your build budget as there most probably will be unforeseen costs. No matter how much you plan and organise there will be a cost that you didn't plan for. Getting out of the ground is generally the time where something will crop up that you couldn't have planned for. 

The design and shape of your build will play a large part in keeping to your budget. The simpler the shape - i.e. a box or rectangle - the better. Once you start adding curved walls, non standard measurements and complex geometric roof shapes the cost of materials and labour will shoot straight up.

Simple doesn't have to second best and can be the most impressive option in many cases.