First of all, the bitcoin price is always determined by the net demand for holding bitcoin. With a given amount of bitcoin available at any point in time, its value must adjust until investors realize their desired allocations, denominated in e.g. USD.
To make a simplified example: If there was only one bitcoin and two investors wanted to hold 1000 USD worth of bitcoin each, that would only be possible with bitcoin valued at 2000 USD a coin.
The current inflation rate is around 3.6% and will drop to 1.8% in May. This means that without a change in demand, the halving should only trigger a 1.8% price increase over the first year after the halving, relative to what would be the case without the halving.