Guys, guys, you're not seeing the full big picture here. Thanks to the way bonuses work, the use of sov resource squares is extremely important for cities WITH VERY HIGH TAX RATES. Let me explain. 

For instance, your city might have 5x L14 farmyards and a L12 flourmill and you have, say, Gift of Nourishment active on the city, and it's summer. The base production is 2860 food, and you have so far a bonus of +32% (24 mill, 5 spell, 3 summer).

With 0% tax rate, your actual food production is boosted another 25% up to +57%, so you get 4490 food. With the 25% default tax rate, no additional bonus granted, you get 3775 food. With 100% tax rate, a -75% penalty bringing you down to 57% of base, so you get 1630 food.

Now, enter a dolmen with 15 food somewhere nearby, and you build it up to L5, so you get 15% from there (for a measly 50 RP if you placed the city right beside it). All of a sudden, the numbers from above become:

0% tax rate, effective bonus +72%, or 4919 food ; 25% tax rate, effective bonus +47%, or 4204 food ; 100% tax rate, effective bonus -38%, or 2059 food.

In all cases, you effectively get a flat +429 food (15% of base) added onto the previous numbers REGARDLESS OF TAX RATE.

EDIT//NOTE : One food *used* at 0% tax rate translates into 4 gold at 100% tax rate.
So that 429 extra food could translate into as much as 1716 extra gold, and you "only" pay 500 gold for it (plus the RP, which would be worth another 12.5 gold plus whatever the extra materials might be worth to you if imported in book form).

A similar logic applies to all other resources too. Later on in the game, you can keep importing a small amount of books from book-production-centric smaller cities with moderate tax rates (decent RP production) to maintain the needed RPs for sov, and ramp up tax rates sky-high in your main city, allowing you to maintain an insanely strong military force (think ~50k basic ranged or ~13k heavy cavalry) without the need to constantly ship anything else in, let alone the massive amounts of either gold or food the city would otherwise need.

But yeah, the bonuses could stand to be a bit higher.

I suppose slightly pre-nerfed and adjusted to be better later on is more desirable than decent early but totally overpowered in later stages and in dire need of a readjustment by the time people have been working up to a setup with those bonuses in mind over weeks or months.

In other words, yeah, building sov resource structures on regular squares is mostly pointless, and it's only on above-average special squares where you get the best out of it.

This makes those few good spots more valuable (and more fought-for), and if you manage to find a concentration of good dolmens and other interesting stuff nearby (like, say, unit production bonus squares), you can almost guarantee there will be some pretty fierce fighting over it in the future, and more fighting is good for suppliers of weapons. 

This also means larger players have an incentive to spread out rather than colonize right next to eachother (for easy defense), so combat becomes noticeably more strategic too.

All in all, proper sov usage is highly situational, and it wouldn't be any fun in the long run if you could just use a cookie-cutter build anywhere at random on the map, now would it ?

One unit of food extra is worth up to 4 gold, since that's the difference between a 0% tax rate and a 100% tax rate income.
Of course, the loss of other resource incomes makes it worth in practice much less than that, but in some cases, its practical worth is pretty close to the maximum theoretical 4 gold.

Population: 26985
That's how much food the largest city in the game right now is consuming per hour (or, at least, one of the largest anyway).
It's on a 5-food square, but has a city on a 7-food square directly to the East of it.

Assuming the 10k base food production quote was accurate for a 5-farmstead city, without prestige, the most he could hope to get out of the city itself is little over 17k food, and that's with 0% tax rate. Even with the prestige bonus, that's still only around 19k food at 0% tax rate... or around 16k with prestige and the default 25% tax rate (giving him almost 27k gold/hour)... or barely over 9k with prestige at max tax rate (giving him roughly 108k gold/hour).

There are also advantages to producing food "locally", namely the fact you don't need to constantly ferry it in, and you're not susceptible to production breaks in case the stockpiles run out after a micro-vacation.

For such a player city, the value of a 20-food dolmen would be roughly equivalent to 8000 gold/hour. You still break-even with it even if you claim it at 16 squares away.

I'm not argueing that some tiles shouldn't be noticably more valuable than others. However if your going to use the argument that valuable tiles will increase fighting and therefore improve gameplay it needs to be noted that at 5 tiles away no square is worth fighting for. The cost for distance factor should be greatly reduced otherwise players will need to make a conciouse decision to contest a tile and make a full committment to that contest due to the close proximity ofthe enemy city