Following great info posted by Deuces {PFJ} on the current LTQ event, I decided to perform a few analyses:
(1) In the table above, it is listed the loot yielded by this LTQ (up to current info). Because I am a boss killer under lvl 75, I am extremely conscious about xp gain. As one can see, the att/xp ratios start out pretty good (above 1), but rapidly decrease to below the average 0.5 att/xp. Note that prizes at these early levels will be mostly irrelevant on the end game though. This time around I settled for lvl 11 with the demolition kit (explosive with 92 att).
(2) The total xp gained for completing normal is 13165 xp. The total attack increase is 6974. The overall ratio for normal is 0.53 att/xp. Note that if you fall short of the final prize of normal just by one level, you'll have a ratio of 0.42 att/xp. Only the last level of the LTQ (either normal or elite) with the final prize, the ratio is dramatically increased (to around 2 att/xp) and will make the entire event pay off. Therefore, one might conclude that it is only worth to pursue the LTQ if one is able to go all the way.
(3) In contrast, boss events are completely different. Killing 50 bosses earns 6625 xp (computed with 10 xp at lvl 1 and 5 xp increases henceforth). The current boss event has 3 grand prizes for a total of 6741/6599. Moreover, there are 50 extra prizes. Assuming an average of 100 (split among lots of commons, several uncommons, and few rares), that factors in with 5000 more. Thus, the rate of stat gain per xp earned with a successful boss killing is 1.78 att/xp, with some luck associated to the random draws of 50 prizes maybe up to 2 att/xp. Even if you fall short of a final prize, it's still an acceptable deal and you'll have a better shot next time if you stay in the same tier.
(4) Lastly, there's the gold usage. I am able to kill bosses for free, I dont need to use gold to go all the way. If you play slowly and are patient, you can do it. Now, normal of the current LTQ costs 40,000 energy to complete. At a regeneration of 3 per minute, that corresponds to 13,333 min = 222.2 hr = 9.26 days. I can factor in maybe 2 or 3 level ups with free energy coming from there, but that's at most a discount of maybe 6,000 energy.
Conclusion: it is impossible to complete the LTQ (even normal) without gold if you dont have insane energy regeneration bonuses and pre-hit the maps extensively (note that pre-hitting doesnt save you xp, though)
Conclusion #2: if you are not going all the way, it is better to just go a few normal levels and then lay off.
Conclusion #3: never before had Crime City an event solely for gold spending (i.e. on which free play is not strategically smart and a rational non-gold spender will not play them at all).
Conclusion #4: even if you spend gold and go all the way to elite, I am not that sure about its true benefit, gold effectiveness, and xp earn.