This need not be the case. Committees play very important roles for organizations of all sizes. This is also true of advisory boards or committees. Like everything in life, you tend to get what you pay for. Advisory boards are no different.
Members can agree to serve for free or they can charge hourly fees.
An advisory board isn't measured by cost but by results. You must ensure that the board is managed to produce these results. Don't have open ended meetings with no tightly focused agenda. The chair must have good communication and leadership skills. The chair needs to be a good facilitator, be well-organized, and have the time to commit to the committee's work.