Creating a brand is always sustainable considering the fact that you are not simply building a brand, it's rather an initiative that has several revenue streams, enhances return sale and lives on it owns equity even though you have no deals and offers this keeps on getting people back. But as the rule of thumb goes no brand is built over nightly or you can't expect to see immediate ROI from your online branding initiative.
Competitive Maturity Index
Running a start up company is difficult in light of bleak revenue sources. Most of businesses run on ideas with little or no operating revenue. Your hope lies with angle investors or your own savings and both are limited as far as you think. Each dollar that you spend must justify it's frugality and how it will earn for the company. Therefore challenge lies with how fast you convert your start up into a self sustaining company.