Transaction value in gst.

No, the government cant say such way.. if your customer is a unrelated party.. No where in the GST law says that Selling price should always be more than Cost price.. Even if questioned, you may justify by showing few sale orders with other customers where SP < CP at same point of time.. 

Thank u sir.

and is there are any caselaws which says that fixed profit peecentage is to be maintained..

There is no Provision under GST Law regarding this matter. You can sell goods even at "Loss". Only be sure that the Invoice is issued timely and in a proper manner as prescribed under Section 31 of the CGST Act 2017 and breakup of GST should be indicated.

Ok sir.

On Sale of Capital goods, section 18(6) provides
[6] In case of supply of capital goods or plant and machinery, on which input tax credit has been taken, the registered person shall pay an amount equal to the input tax credit taken on the said capital goods or plant and machinery reduced by such percentage points as may be prescribed or the tax on the transaction value of such capital goods or plant and machinery determined under Section 15, whichever is higher

but no such provision relating to goods .

Penetration of market (competition) should be considered as extra consideration.(Fiat India Ltd case).
Department can consider it as underbilling, so to my mind it is better to pay GST at least on cost.

Sir can u provide case law details...

The case law was decided by the honourable supreme court and pertains to excise time period. The ruling was not overruled by the government and it was merely rendered ineffective by changing the law itself. The judgement is still alive and applicable even in GST era.

Very apt mention by Pioneer Proffessionals