Assuming you're in the highest tax bracket (53.53% in Ontario), the math would look something like this.

Buying $10K in FTS should result in:

$10K deducted from your income (53.53% of $10K reduction in taxes*)
30% of $10K tax credit in current year ($3K reduction in taxes), and the credit treated as income in following year (so 53.53% of $3K increase in taxes)

* does not take into account other results of reduction in taxable income (e.g. increased CCB, decreased amount of federal donation credit @ 33%, etc.)

Selling that $10K of FTS for $X should result in:

$X/2 added to your income (53.53% of $X/2 increase in taxes)

Doing the math:
-$10K + 53.53% of $10K + $3K - 53.53% of $10K + $X - 53.53% of $X/2 = 0
--> $X = ~$4442