It's entirely possible for halving to be priced in now.
When you buy bitcoin from an exchange (the action that determines bitcoin price) you aren't buying from miners, you're buying from bitcoin holders. Sure, that could INCLUDE miners, but it's not exclusive.
How non-exclusive is it?
According to bitcoinity.org market charts, every day people buy between 1 million and 1.3 million BTC on the exchanges it tracks. If we exclude Houbi and OKcoin (as their numbers are likely inflated) that number falls dramatically to something closer to 200,000 BTC per day. But Houbi and OKcoin are probably doing greater than 0 volume, so let's just say, conservatively, 300,000 BTC are purchased on exchanges every day.
Miners produce (on average) 3600 new BTC per day. If we assume that every miner everywhere sells 100% of his coins immediately, then mined coins represent 1.2% of the supply. If they hold any amount of those coins, than that number is even less.
As we all know, price fluctuations are a result of the ebbs and flows in the bitcoin supply versus demand. So the argument that the reward halving is going to reduce the supply and thus increase the price of bitcoin is indeed valid, but let's not get carried away with 50->100% price increase projections. Remember that people holding coins are not going to have their coins halved, and they represent 98.8% of the daily supply.
So what will happen to the supply (strictly as a result of mining reward halving)?
Well, we're at ~300,000 now, which includes a 3600 mining reward. If we assume once again that all miners everywhere sell 100% of their coins, the biggest impact we could see is a drop of 1800 BTC on the supply side, bringing us down to ~298,200 (or -0.6%).
If everything else stays the same, this would result in a very modest price incline. The deficit is cumulative every day, so in theory the price would have to inch upward to make up for this new 0.6% discrepancy in supply and demand until a new equilibrium is found, but that would not be discernible from normal price movement in all likelihood.
The catch here is that everything else is NOT likely to stay the same. As you can see, there are people that believe that the bitcoin supply will be halved, or that the price will increase 100% immediately after the halving. These beliefs create demand. Demand also rises the price. So while almost nothing is happening on the supply side, we MAY see more significant movements on the demand side.
So why might the halving be already priced in then?
Well, the halving is an event that represents a 0.6% (max.) reduction in the bitcoin supply, something that would warrant a very modest price rise. Meanwhile people have been talking about it and anticipating it for over a year. This means that over that course of time, demand has risen as a result of speculation. Remember that price is set as a function of supply versus demand. Supply going down has the same effect as demand going up in regards to the price. Therefore it can be assumed that any price change that would have happened as a result of the supply side shrinking would have been assumed the moment it could be anticipated on the demand side, which is always immediately with bitcoin.
This doesn't mean that the price WON'T rise immediately before or after the halving. As we saw recently with litecoin, misinformation and hype are powerful (albeit temporary) forces on supply and demand. This is always a possibility, but this would not be a result of the actual halving, only people's perceptions on what a halving will do to the market. After the hype and misinformation phase subsided, the actual supply and demand found it's equilibrium once again roughly 12-14% higher than before the halving (priced in BTC).