Answer:
The total interest cost is 110, 602 dolars.
Explanation:
The total interest cost has been calculated below. The total interest cost is calculated by multiplying interest rate per month with outstanding loan balance.
Key
M means month there are 240 months in 20 years = 20 *12 =240
Os outstanding loan amount
P payment means monthly installment (principal + interest)
I mean interest per month (7%/12) 
PP means payment towards principal
Bal outstanding loan balance = loan amount -principal paid
M       OS            P       I              pp         Bal
1	128,500	996	749.58	246.68	128,253.32
2	128,253	996	748.144	248.12	128,005.21
3	128,005	996	746.697	249.56	127,755.64
4	127,756	996	745.241	251.02	127,504.63
5	127,505	996	743.777	252.48	127,252.14
6	127,252	996	742.304	253.96	126,998.19
7	126,998	996	740.823	255.44	126,742.75
8	126,743	996	739.333	256.93	126,485.82
9	126,486	996	737.834	258.43	126,227.40
10	126,227	996	736.326	259.93	125,967.46
11	125,967	996	734.810	261.45	125,706.01
12	125,706	996	733.285	262.97	125,443.04
13	125,443	996	731.751	264.51	125,178.53
14	125,179	996	730.208	266.05	124,912.48
15	124,912	996	728.656	267.60	124,644.87
16	124,645	996	727.095	269.16	124,375.71
17	124,376	996	725.525	270.74	124,104.97
18	124,105	996	723.946	272.31	123,832.66
32	120,145	996	700.843	295.42	119,849.18
180	51,011	996	297.567	698.69	50,312.80
181	50,313	996	293.491	702.77	49,610.03
182	49,610	996	289.392	706.87	48,903.16
183	48,903	996	285.268	710.99	48,192.17
184	48,192	996	281.121	715.14	47,477.03
185	47,477	996	276.949	719.31	46,757.72
186	46,758	996	272.753	723.51	46,034.21
187	46,034	996	268.533	727.73	45,306.49
188	45,306	996	264.288	731.97	44,574.51
189	44,575	996	260.018	736.24	43,838.27
190	43,838	996	255.723	740.54	43,097.73
191	43,098	996	251.403	744.86	42,352.88
192	42,353	996	247.058	749.20	41,603.68
193	41,604	996	242.688	753.57	40,850.10
194	40,850	996	238.292	757.97	40,092.14
195	40,092	996	233.871	762.39	39,329.75
196	39,330	996	229.424	766.84	38,562.91
197	38,563	996	224.950	771.31	37,791.60
198	37,792	996	220.451	775.81	37,015.79
199	37,016	996	215.925	780.33	36,235.46
200	36,235	996	211.374	784.89	35,450.57
201	35,451	996	206.795	789.47	34,661.11
202	34,661	996	202.190	794.07	33,867.04
203	33,867	996	197.558	798.70	33,068.33
204	33,068	996	192.899	803.36	32,264.97
205	32,265	996	188.212	808.05	31,456.92
206	31,457	996	183.499	812.76	30,644.16
207	30,644	996	178.758	817.50	29,826.66
208	29,827	996	173.989	822.27	29,004.39
209	29,004	996	169.192	827.07	28,177.32
210	28,177	996	164.368	831.89	27,345.43
211	27,345	996	159.515	836.74	26,508.68
212	26,509	996	154.634	841.63	25,667.06
213	25,667	996	149.725	846.54	24,820.52
214	24,821	996	144.786	851.47	23,969.05
215	23,969	996	139.819	856.44	23,112.61
216	23,113	996	134.824	861.44	22,251.17
217	22,251	996	129.799	866.46	21,384.71
218	21,385	996	124.744	871.52	20,513.20
219	20,513	996	119.660	876.60	19,636.60
220	19,637	996	114.547	881.71	18,754.88
221	18,755	996	109.403	886.86	17,868.03
222	17,868	996	104.230	892.03	16,976.00
223	16,976	996	99.027	897.23	16,078.76
224	16,079	996	93.793	902.47	15,176.30
225	15,176	996	88.528	907.73	14,268.56
226	14,269	996	83.233	913.03	13,355.54
227	13,356	996	77.907	918.35	12,437.18
228	12,437	996	72.550	923.71	11,513.47
229	11,513	996	67.162	929.10	10,584.38
230	10,584	996	61.742	934.52	9,649.86
231	9,650	996	56.291	939.97	8,709.89
232	8,710	996	50.808	945.45	7,764.44
233	7,764	996	45.293	950.97	6,813.47
234	6,813	996	39.745	956.51	5,856.96
235	5,857	996	34.166	962.09	4,894.86
236	4,895	996	28.553	967.71	3,927.15
237	3,927	996	22.908	973.35	2,953.80
238	2,954	996	17.231	979.03	1,974.77
239	1,975	996	11.520	984.74	990.03
240	990	996	5.775	990.48	-0.45