Some trainers feel uncomfortable charging one client one fee and another client another fee. Somehow it seems unfair, and there's a feeling that if a customer finds out someone else got the same service for a lower price, that hard feelings will result. Those can be valid concerns, but there's another side to this.
Because seminar fees vary so much both between and within any geographical areas, it's difficult to charge a constant fee to all customers. In addition, consider that clients ability and willingness to pay also varies. A huge corporate client may be comfortable paying $5,000 per head for executive training but a small business owner probably won't be willing or able to pay that level of fee.
Since I deliver seminars in different areas, and I work with different kinds of organizations, I usually ask the potential client what they are used to paying, explaining that rates vary, and I want to find a price point that works for them. I've never had a problem negotiating on this basis. This is a customer-focused strategy.
On balance, it's probably good to use "flexible pricing" to accomodate the differing needs and finances of your various potential customers but do so with reason. You can be flexible, but if you end up charging one customer $10,000 for a service, and the next customer $1,000, that's problematic. Smaller variations are OK.
Consider charging non-profits or charitable organizations on a different basis, if you want their business.
Again, this is a personal decision trainers need to make, but on balance, flexibility is useful.