Thanks, but let's use as an example a simple flight to Sydney from Melbourne, where it takes 8000 Qantas FF for an award flight for one to Sydney, plus $31 taxes and fees.
So, using the Amex card with 2.25% surcharge and earning two points, I'd have to buy 4000 litres of petrol at say $1.44 per litre (with my 4 cents a litre discount) and pay the 2.25% surcharge. Total cost of flight = $5889.6 +$31
Using a card with 1 point per dollar and no surcharge, requires 8000 litres. Total cost of flight =$11,520 + $31
Using a card with half a point a dollar an no surcharge, requires 16,000 litres. Total cost of flight = $23,040 +$31
But using no card, and not buying any petrol at all. Total cost of flight = $129 to $199
Firstly, I never compare actual cash value to CC spend - its a convenience thing - so regardless if I could save a further 1% by using folding money I know I won't ever go to the trouble, therefore I add the cost of using a card to my calculations (and its never more than 1% as I won't pay more then that ... will walk away if required).
Additionally, I assume that whatever I'm buying is something that I need or want to buy, the money for petrol, in the example above, will be spent on petrol regardless of other factors. In the above example the Amex is out as it costs 1.125c/point. The other two have no surcharge so they are 'in'. Like a lot of FF chasers I have a lot of credit cards, pretty quickly I came to realise that I couldn't easily, at the point of sale, remember which cards had what bonuses and so-on, thus my reliance on my self instituted maximum cost of no more than 1c/p. I assume that each point gained costs me 1c. If its free, well, great, more win for me.
So, I can't really value the points in the manner that you do. Rather, the only way they can really be valued (assuming I am willing to pay for Qantas's service and safety record) is
($129-$31)X100/8000points = 1.225 cents per point
to
($199-$31)x100/8000points = 2.1 cents per point
Usually there is an option with rewards flights to pay the total amount, including fees and taxes with points, I declutter my calculations by assuming points are money and money is needed as a total to gain a ticket. A big part of the reason for this is that different airlines charge wildly different fees (fuel surcharges and whatnot). So, in my calcs, this particular ticket is going to cost me $129 in folding money, how many points is that?
$129 = 8000 points + some more points for the tax component (I'd have to check, but prolly around 6000 ... is that about right?) = $129/14,000 = 0.009 or 0.9c/point - in my world, I wouldn't do it, as I'm always assuming my points cost 1c/p and are worth more (or at least I need to convert them for more to make this a decent business proposition)
$199 = 8000 points + 6000 points (tax) = 199/14,000 = 1.4c/p
Simplistically speaking.
However, it doesn't stop there:
The purchase of the flight outright with money (cc) will gain me 129 points if the earn is 1:1 and the flight itself will gain me some points too, 490 with VFF and I think 800 (minimum points guarantee??) with QFF. Going with the QFF example, 129+800=929 points earned if paying card. Value of those points at minimum is $9.29 (I won't convert for less than 1c/p as previously mentioned).
So, modified cash price becomes $119.71 (inc tax) (status credits also have value, so you might modify further, but thats another conversation)
Points cost = 14,000 all inc, and at 1c/p = $140 ... and for a lot of airlines there is no FF point earn on straight rewards seats.
Obviously I am using my own valuation of points in the example above. You can back convert to arrive at a figure which suits you - but be strict with yourself. If your own valuation comes out at 2c/p then never pay more than that as its a losing proposition. Likewise, if you back convert a few example rewards seats/cash costs and can't stretch to more than 0.5c/p then never pay more.
Whilst the single example you have given shows are marked difference in potential points value (1.225c/p versus 2.1c/p) you will almost certainly find that this difference diminishes to almost nothing once you add a few more data points to the stats - do the same calculation for the same route (if this is one you travel frequently) over say 6 months if its relevant to your flying habits.
Worst case, and using only one data point, you can simply go with 1.225c/p ... don't pay more and you can't lose, likewise, don't exchange points for less than 1.225c/p