OK, here's a little maths lesson for you.
The current rates of SDLT are given on the gov.uk website here:
https://www.gov.uk/stamp-duty-land-tax/residential-property-rates
There is even an official government SDLT calculator there as well (which, spoiler alert, gives the same figures that I do).
The current rates are:
Property value SDLT rate
Up to £125,000 Zero
The next £125,000 (the portion from £125,001 to £250,000) 2%
The next £675,000 (the portion from £250,001 to £925,000) 5%
The next £575,000 (the portion from £925,001 to £1.5 million) 10%
The remaining amount (the portion above £1.5 million) 12%
So, if you are buying a house for £450k (assuming that you will not be owning two or more properties) then your stamp duty is:
0% on the first £125k = £0
2% on the next £125k = £2,500
5% on the final £200k = £10,000
Total = £12,500
.
If you are buying a house for £600k then your stamp duty is:
0% on the first £125k = £0
2% on the next £125k = £2,500
5% on the final £350k = £17,500
Total = £20,000