For a mobilehome which is to be sited in a mobilehome park as defined in Section 18214 of the Health and Safety Code, the department shall charge a rate of interest higher than that which is charged to purchasers of conventional housing or of a mobilehome sited on a lot owned by the purchaser. The interest rate shall be set by the department to balance lending risk while maintaining a rate at or below the market rate for a mobilehome in a park.