In general you are right. If you go and try to lay at SP for $10,000 and there is only $4K being bet on SP then they are going to look at the exchange to work out how to price the SP odds for the horse. If there was $2K at $100 and 2K at $500 and then $2K at $800 then your SP price is going to blow out to the $800 mark.

It all depends on liquidity. The more you lay the higher the price will be that you lay at. Its kind of like the tote but not quite as severe.

Mind you 10K to lay at $200 is only $50 for someone to bet so I am not surpirsed you can easily lay the $10K.