Fellow speaker,
If you speak for a fee, keep that fee consistent. Offering an extra cost if using a speakers bureau and offering a discount when not using these outside agencies may work to get you personal bookings (non-speakers bureau bookings) for the short term. But when the speakers bureau hears about it (and they will), and spreads the word that you use speakers bureaus only to sell yourself direct at a lower cost to the client, they will spread bad words about you...and this could affect your current clients if they do business with speakers bureaus.
Consistently,
Tim