54 Lethality is not 54% extra damage. It removes 54 armor from the target during the damage calculation.
Assuming you do not make it so they have negative resistances during the damage calculation, the damage formula is 100/(100+armor). However, lethality will not make it so the target can have negative armor, only armor reduction items can according to the wiki. Using the wiki, we can also assume most champions will be between 75-110 armor without any bonus armor at level 18. Knowing this info we can do some math.
Base Stats:
100/175=57.14% damage taken
100/210=47.61% damage taken
Tabi:
100/195=51.28% damage taken
100/230=43.47% damage taken
Tabi+Seeker's Armguard(Fully Stacked):
100/240=41.66% damage taken
100/275=36.36% damage taken
Calculating in the 54 lethality:
100/121=82.64% damage taken
100/156=64.10% damage taken
Tabi:
100/141=70.92% damage taken
100/176=56.81% damage taken
Tabi+Seeker's Armguard(Fully Stacked):
100/186=53.76% damage taken
100/221=45.24% damage taken
Differencing the two columns:
82.64-57.14=25.50% difference in damage
64.10-47.61=16.49% difference in damage
70.92-51.28=19.64% difference in damage
56.81-43.47=13.34% difference in damage
53.76-41.66=12.10% difference in damage
45.24-36.36=8.88% difference in damage
So 54% is not even close to realistic here. Furthermore, you poorly recognize the value of the stats from the ADC build. Infinity Edge alone makes it so you can crit for 225% of your base damage. Assuming the ADC is level 18, they will have between 95-137 base AD. Since crits make you do 225% damage, you have the potential to gain between (80+95)*2.25-95=298.75 AD and (80+137)2.25-137=351.25 AD from items. So there is a 75% you gain 298.75-351.25 AD from items and a 25% chance you only gain 80 AD. If I am not completely messing up the math, we can average this by doing (3Crit damage + 80)/4. This gives an average damage increase of 244.0625-283.4375 AD. On top of all that extra damage, ADCs also getting a whopping 65% attack speed.
So let's compare, lethality can give 165 AD and remove 25.50% damage reduction in a best case scenario. Additionally, asssassin items give 20% CDR and 250 Hp. On the other hand, ADC builds can 244.0625-283.4375 AD on average along with 65% attack speed. Even assuming you get exactly 165*1.255= 207.075 AD from lethality, the AD from an ADC build is far greater for 300 gold more. And we'll just call the other stats close enough.
Edit: Calculating gold efficiency. Using the wiki, we find values for gold efficiency of AD, Lethality, and AS.
Assassin build:
165 AD = 35x165 = 5775
54 Lethality = 22.5x54 = 1215
20% CDR = 26.6x20 = 532
250HP = 2.6x250 = 650
Total Value = 8172
8800 cost
Gold Efficiency = 92.86%
ADC Build:
244.0625 - 283.4375 AD = 35x244.0625 OR 35x283.4375 = 8542.1875 OR 9920.3125
65% Attack Speed = 25x65 = 1625
Total Value = 10167.1875 OR 11545.3125
9100 cost
Gold Efficiency = 111.60% OR 126.87%