Like everything ... it depends. Generally if you are being paid for and paying for (ie have customers and are paying employees salaries) you have to manage some combination of what you are best able to use as a tool and what your customers demand. 8k is not a lot it is about 1 developer month worth of expense. So if you are building anything that you expect to use more than say 6 people months developing the SQL cost is relatively trivial.

If you are a small shop/just starting out try some combination of SQL Express, Azure, or getting hooked up with BizSpark. By the time you need to scale up you should have the revenue to do so. It is all what you as a developer and your customers can support in house. In my experience the expectation is that MySQL/PostGres will be a free/cheap offering and anything on Sybase/DB2/MS SQL etc (the enterprise dbs) will be at "professional" prices. Part of that is they have the built in expense of the database license but I think a part of that is customers mindset IMO too. Give me a computer department ticketing system running on MySQL and I'll assume it is FOSS or try to find a free one. Give my one running on SQL and I'll expect to spend $2000 a year and have someone to call that one time in 10 years I need customer support. If the problem it solves is worth the cost I don't care one way or another make the problem go away and I'll be happy. I suspect most customers are that way too (and the ones not willing to pay a profitable amount for the development cost of the software aren't really customers worth having are they?).