I'm wondering if there can actually be a definitive answer to your question anyway, Grumpy.
The 'starting price' for a horse is determined either by consensus among on-course bookmakers or by averaging the odds offered by them. I suspect that in the 18th century, on-course bookmakers will have operated largely independently of each other and that nobody will have collated the odds offered by different bookmakers, meaning that there was no such thing as an 'official' starting price in those days.