Never be afraid to negotiate with anyone. If the only thing standing between you and a inviting a great motivational speaker to your event is your budget, the potential speaker may be willing to negotiate the difference with the following negotiation tactics:
1) Deal directly with the speaker's office. If you call a speakers bureau or another third party they will charge a commission to the speaker; leaving less room in the budget to negotiate.
2)Consider what's in it for the speaker: an audience of decision makers, potential repeat engagements, a really warm and receptive crowd that will buy their books, follow them on twitter and tell all their friends and family members....offer to refer guest speakers to national associations or other professional groups, promote them in your industry publications. Look for ways to exchange value for value.
3) Make it easy for the speaker. If possible, book your meeting around when the speaker is in town for another motivational talk. Make the logistics easy, pick guest speakers up at the airport, book the hotel room. It is usually the small gestures that make a big difference.
4) Book multiple engagements with the same motivational speaker and you may be able to get a price break. Also look for ways to really utilize the speakers talents by having him do a keynote and a breakout session for the same fee.
5) Introduce the Keynote speaker to a Sponsor. Sponsors are looking for opportunities to promote and align themselves with alliance that fit. A keynote speaker who specializes in their industry, is known by the sponsors target market and/ or has a message in line with a sponsors goals may just be a fit and offer the speaker opportunity for multiple engagements.
It will help if you have a budget for your keynote speaker in the first place. This give you a perimeter to work with. If you can't afford a motivational speakers fee, than send them another offer suggesting the maximum investment you can afford and all the other benefits ( above) of presenting to this audience.
Also consider writing a thank you letter, offering to do a nice testimonial for the speakers website, publishing a follow up article about the speaker and their motivational talk. All of these gestures help leverage the experience for the speaker and offer leverage for you in negotiating the fee.