The theoretical "value" of BTC post-halving is double what it is right now so around $860. That's the VALUE and PRICE may and probably will be different, because 1) the market is not fully efficient and 2) the halving has already probably been taken into account with our "doubling" pump from $220 -> $440. This is assuming demand for BTC remains the same.
What will probably happen at the halving is whales will SELL the news against noobs who are just discovering about the halving and going fomo. Then over time it will pick up again.