The more expensive plan allows monthly calculation, the less expensive is a year's commitment. Note: you MUST CANCEL in time. You cannot take the plan for 3 months, in advance.
You can get the Photographers Plan, which includes Lightroom as well as Photoshop, it's a one year commitment, but it costs only around $10 a month. Alternatively, there's the cutdown version of Photoshop, called Photoshop Elements, for a one-off payment of around $70.