I'm using monotouch, so the names and code will be a bit different, but you can do this by making sure that the width of the collectionview equals (x * cell width) + (x-1) * MinimumSpacing with x = amount of cells per row.
Just do following steps based on your MinimumInteritemSpacing and the Width of the Cell
1) We calculate amount of items per row based on cell size + current insets + minimum spacing
float currentTotalWidth = CollectionView.Frame.Width - Layout.SectionInset.Left - Layout.SectionInset.Right (Layout = flowlayout)
int amountOfCellsPerRow = (currentTotalWidth + MinimumSpacing) / (cell width + MinimumSpacing)
2) Now you have all info to calculate the expected width for the collection view
float totalWidth =(amountOfCellsPerRow * cell width) + (amountOfCellsPerRow-1) * MinimumSpacing
3) So the difference between the current width and the expected width is
float difference = currentTotalWidth - totalWidth;
4) Now adjust the insets (in this example we add it to the right, so the left position of the collectionview stays the same
Layout.SectionInset.Right = Layout.SectionInset.Right + difference;