How how much punya does the father get from a kanya dana?
What is the phala for a kanya dana?
I have heard that it gives the father the Brahma Loka. But, the references I have collected only say that it guaranties the father a place in the celestial region. Also, it bestows merit equal to performing a Jyotistoma Vedic Yajna.
Following verses are from the Samvarta Smriti:
He, who, decorating a maiden with ornaments, gives her, unto a becoming bridegroom, according to the Brahma mode of marriage', enjoys extraordinary prosperity by the virtue of the giving away of the maiden, acquires the applause of the pious, and attains to un-ending fame. (61,62)
Having given away [a maiden], sanctified with Homa Mantrams, a person attains the fruits of hundreds of Jyotistoma-Sacrifice. (63)
Having given away a maiden-daughter, decorated with ornaments, clothes and seats a father attains to the celestial region and is adored of the Suras (celestials) . (64)
Regarding the Brahmaloka Prapti, I have found a mantra recited during the ritual. This website has the mantra and it says the mantra is from Smritiratna (which is a Smriti digest).
According to Smrtiratnam, the following mantra is to be recited at the time of performing kanyA dAnam during vivAha :
kanyAm lakshaNa samyuktAm kanakAbharaNairyutAm|
dAsyami vishNave tubhyam brahmaloka jigIshayA ||
Desiring to reach the abode of Brahma, I give as kanyA DAnam the bride with good features and decked with golden ornaments to you (a deserving groom) who is VishNu svarUpa
The procedure of Kanya Dana is mentioned in great details in the book "Essence of Dharma Sindhu". The Sankalpa mantra clearly states the purpose of the Dana or the fruit to be achieved in return.
Kanyaa Daana Prakriya:
The Jyotirvetta would rearr ange the seating arrangement as a preliminary to t he key act of Kanyaadaana; the groom would face ea st and the bride would face west while the Kanya d aata and his wife would be seated on the southern side and having taken ‘Kushaas ‘in hand accomplis h the act of Kanya daana as follows :
[Mamopaatta duritakshaya dwaraa Parameshwara preetyartham shubhey shobhaney muhurtey Shri Vishnoraagnayaa Adya Brahmanah Dwiteeya ParaardheyVaivaswata Manvantare y Kali yugey pradhamey paadey Jambu dwipey Bharata VarsheyBharata Khandey Meror dakshina digbhaagey Aryaavartey or Shri Shailasya Ishanya orVayavya o r Agneya pradeshey etc. Samastha Devataa Brahmana Guru jana sannidhou Asmin Vartamaana Vyavahaarika Chaandra maanena or Surya maanena --- Samvatsarey, --Ayanye—Rutou,--- Maasou,--Pakshou—Shubha Tithou —Shubha Vaasarey—Shubha Nakshatra---Shubha yoga-Sh ubha Karana Evam guna visheshana visishtaayaam] Am uka Pravaraamuka Gotromuka Sharmaaham Mama Samasta Pitrunaam Niratishayaananda Brahma lokaavaaptyaad i Kanyaa daana kalpokta phalaavaaptaye Anena Varen aasyaam Kanyaamutpaada ishyamaana santatyaa Dwadas haa Varaan Dwadasha paraamscha Purushaan Pavitrika rtum Atmanascha Lakshmi Narayana preetaye Braahma Vivaaha vidhinaa Kanyaa daanmam karishye!
( After announcing the details of the Desha kaala details on the lines of [----] above, the ‘Kanya Daana Kar ta’ would recite the following : I, of Amuka Prava ra Aamuka Gotra Amuka Sharma, am performing this K anya Dana so that this auspicious act would faci litate the Phala prapti to let my Pitru Devaas to attain lasting happiness and Brahma loka Nivasa as also to pave way for the prospective generations out of the union of these Vadhu-Varaas to sanctify me and my wife as also twelve brides and twelve g rooms in future besides pleasing Bhagavan Shri Lak shmi- Narayana !
So, the Phala is not just Brahma Loka prapti but it is the main fruit.
In addition to Rickross' answer, I have found other verses that illustrate the punya phala of a kanya dana:
Viṣṇu (24.34-37).—‘He who gives his girl in marriage by the Brāhma form goes to the regions of Brahman; by the Daiva, to Heaven; by the Ārṣa, to the regions of Viṣṇu; by the Prājāpatya, to the regions of the gods; and by the Gāndharva form, one goes to the region of the Gandharvas.’
Dakṣa (Vīramitrodaya-Saṃskāra, p. 866).—‘The reward accruing from the marrying of a girl by the right form is double in the case of her being given to a Brāhmaṇa; a hundred-thousandfold in that of her being given to a learned Brāhmaṇa; and endless in that of her being given to a thoroughly learned Brāhmaṇa.’