The value of £600 in 1702, would be £76,600.00 today, using the retail price index, or £1,090,000.00 based on average earnings.
I doubt if I could get such an advance on Mrs. Gulliver’s Secrets. [once the book will be published, the previous sentence would read: I didn't get such an advance on Mrs. Gulliver's Secrets].
It's also interesting to note that Gulliver got this huge advance in 1702 while the book was ready for publication only in 1726. I doubt if there was ever a more patient publisher in history.