how can we quantify the effect of simply having *some form* of security...?
Brief aside; of course we should have some security. I'm only saying that the things TSA has done are generally both unnecessary and ineffective.
quantify the effect of ... security to deter the less-suicidal ones?
The way to measure the deterrent effect of a system is by looking at the risk in cases where that system is not in place. In the case of TSA, we can look at cases where the TSA has no deterrent effect and there isn't an analog agency or system. By looking at the probability of attacks that are not deterred by TSA, and comparing that to the probability of attacks in cases where TSA is in place, we can approximate the risk mitigation.
So, for example, we have little or nothing like TSA to deter toxic gas attacks in crowded public spaces. AFAIK, the recent (and possibly accidental) chlorine gas release at the furry convention is the only case since the creation of TSA.
Consider how much terrorism we experienced in the recent years prior to TSA, the level of terrorism in first world nations that don't have something like TSA, and the number of events in the US in areas that are not protected by TSA or a similar deterrent. Compare that to the three minor terrorist attempts that made it through TSA's watch, and their threat level. Even if you take the most pro-TSA estimates you reasonably could, I think we're talking about a deterrent effect that falls somewhere below the life saving benefit of "Don't Run Near The Pool" signs -- at a much higher cost in both dollars and liberty.