I have got one clue from Vishnu Purana! According to Vishnu Purana, before the division of single Veda into 4 (and further division of four Vedas into different Shakhas by Vyasa's chief disciplines), the total volume of Veda was 100000 (1 Lakh) verses
Quoting Vishnu Purana 3.4
पराशर उवाच ।
आद्यो वेदश्चतुष्पादः शतसाहस्त्रसम्मितः ।
ततो दशगुणः कृत्स्नो यज्ञोऽयं सर्वसामधुक् ।। १ ।।
English Translation by H.H.Wilson
The original Veda, in four parts, consisted of one hundred thousand stanzas; and from it sacrifice of ten kinds, the accomplisher of all desires, proceeded.
And the volume of 4 Vedas found today is :
- Rigveda - around 10,600 verses
- Samaveda - around 1,700 verses
- Yajurveda - around 3,000 verses
- Atharvaveda - around 6,000 verses
Total of 21,300 verses (round figure) are surviving today.
So, as per this calculation, 21% of whole Veda of 10^5 is surviving today and 79% of Veda is lost.