Building a new concept based on open source software can be very affordable. Start with planning, start at a high level of the funnel… do a drill down per phase (sign-up, log-in, product page, etc.), then once that’s worked out a high level, comes mock-up time (see Balsamiq as a great affordable solution) (I work with Amit Daliot who makes Axure protopying, about $2000, depending on complexity). Get a solid designer ($500). Final step: theming, I just love PSD2HTML (+$1500).
The video below shows the first phase of getting it done.