Here's an example how you can use Instant Member to create a special priced trial membership:

To create a membership that bills at $7 and thereafter $27.00 you need to be running version 1.2 of Instant Member.

Go To Manage Membership Level > Add New Membership Level

Then the following:

Payment Type:  Subscription
Membership Period: 1 Month
Payment Gateway:  PayPal
Membership Amount: $27.00

Offer Trial Membership:  Yes
Trial Period:  1 Month
Trial Amount:  $7
Subscription Type:  Until Cancelled

This will create a subscription as follows for your members:

$7.00 USD for the first month - Then $27.00 USD for each month

Of course you can choose any prices for the trial (including free) and the regular monthly payment.  And you could also set the trial period for a longer or shorter period to suit your own requirements