There are a number of ways to express this and which way is preferable will depend upon various factors, such as the number of the party offering thanks and the point of view from which thanks are offered (among other things). To express 'thank you', Latin usually employs some indicative form of 'agere' (or of a similar verb, as, e.g., 'reddere') with 'gratias' (less frequently 'grates') as accusative of direct object and 'tibi' (singular) or 'vobis' (plural) as dative of indirect object. The reason for rendering thanks may be expressed by the prepositions 'pro' or 'de' + the ablative or 'ob' or 'propter' + the accusative; the precise choice of preposition will depend upon the nuance intended. Thus (to list only representative possibilities) one may say:

Tibi gratias pro auxilio tuo ago.
Tibi grates propter auxilium tuum ago.
Vobis gratias de auxilio vestro agimus.
Vobis grates ob auxilium vestrum agimus.